The pending release of Tasha, and the multiple threads discussing psionics, had me reminiscing about the psionics system that was presented in the appendix of the AD&D PHB (1978), which builds upon and codifies the earlier appearance in OD&D (Eldritch Wizardry).
But not just about psionics! Psionics can be a divisive topic; some people don't like psionics because they don't want "sci-fi" in their D&D (don't cross the streams!). Others don't like psionics because the rules have often been over-complicated or difficult to implement. Still others don't like psionics because they have often been added, or additional rules that come after the base rules (in an appendix, or in a separate guide) and it's never neatly integrated into the standard D&D adventuring model .... it's hard to account for the psionics when, for example, most published material isn't taking them into account.
Instead, I was marveling at how complicated and fiddly the AD&D psionics system was. And I contrasted that to the raging debates people were having over psionics in 5e; that the mystic or a dice pool was too complicated (for example) and that's why they went to a spell system.
...Complicated? The mystic and a dice pool was complicated? HA! Back in the day, we had to deal with casting times and weapon speed factors just to determine when your first level illusionist would die so you could sit in a corner and read Dragon Magazine the rest of the afternoon. Don't talk to me about complicated.
But no, that's not what I was thinking about. Instead, I was thinking about the difference in approach to design embodied by the two systems.
AD&D is a codification and expansion of OD&D; a set of rules that arose organically, and was customized to individual situations.
This is the bespoke suit. The suit is tailored to you, now. If you gain weight, lose weight, or someone else wants it, too bad. But boy, it looks sharp!
5e prefers simplified and universal resolution mechanics.
This is the leatherman multi-tool. It may never be the best tool for any particular job, but you know you can use it for most things.
I thought I'd briefly explore why I think that, and then have a conclusion explaining why I think this leads to the rampant "spell equivalency" design model in 5e.
A. It is better to look good than to feel good.
One of the most entertaining thing about getting five OD&D/AD&D players together, is that you'll get six opinions about how the rules work. Partly this is because of the amount of DIY as well as table and regional variance in interpreting and applying the rules. But partly it is because the rules and the rule subsystems are confusing and contradictory at times. The rules might say no resurrection for elves, because they have no souls. But the DMG might say, "Hey, the rod of resurrection works on elves." And if you want to know about how surprise works, well ... there's a page in the DMG that explains it, but it's wrong, and certain things have different rules maybe (Rangers and some monsters for example) and dex counts, except when it doesn't, and some monsters have dex (but most don't), and certain items and monsters determine surprise on different dice except that surprise is rolled on a d6 because it can effect the segment count in a combat and ... ahem.
The point is certain things can get complicated. And the reason for that is that AD&D (1e) essentially is a codification and expansion of OD&D and articles from The Strategic Review and Dragon Magazine. And as the game was growing, and situations came up, rules were added. A lot of the these rules made sense for the particular situation that they were being applied to! They were created because they "felt right," and in isolation, they do. If you need to create a new rule to handle, say, grappling and pummeling because you wanted some non-lethal bear hugs and hit, you just created a subsytem for it ... that involved percentiles, and math, and then you needed to break out the tables, wait, is guy wearing a helmet? Oh man ....
So there is a reason that this system is appealing; it grew organically. It often has satisfying solutions for individual rules issues (grappling being a notable exception). And it allows for a lot of flexibility in terms of abilities and differentiation; after all, if everything is custom, what does it matter if you add something new?
B. Teamwork means you always have someone to blame.
5e, on the other hand, is very much more modern in its design. Unlike 1e, you can be reasonably certain that most things can be resolved through the application of a d20 roll, instead of wondering if this will be a d6, d20, d00, or something else entirely. Attacks, saves, ability checks, and skill checks are standardized and roughly equivalent. While people can (and do) argue about rules interpretations, often because of issues related to either the use of natural language (various uses of 'melee attack' or 'attack' or 'unarmed strike' or 'melee weapon attack' etc.), or the intersection of "fluff" and "crunch" (how big of a mess does it make when a druid explodes after putting on metal armor).
But regardless of some complications (....hiding....stealth....), 5e is designed to be simple, robust, and flexible.
Side note- this isn't a Rod of Lordly Might measuring contest with other TTRPGs. It is entirely possible for other games to be more simple, more robust, or more flexible. We are just discussing D&D in this post.
This means that instead of creating multiple new and different subsystems to deal with variant issues, it is expected that you will use the systems provided within the world. You don't need a new subsystem for dealing with each and every animal you encounter; you simply apply the rules for "animal handling" with appropriate DCs and/or modifiers. And so on. Even when presenting a whole new section of rules (such as ships and water combat, as in Ghosts of Saltmarsh), the usual approach is simply to apply the current rules with a few extra guidelines (give ships initiative, give crews a quality score, and proceed on with the usual rules that tables are familiar with).
....and I would assert that this approach has an effect on the design of classes and player options as well.
C. My next trick will be to be a really happy guy. I'll just go around on the internet being really happy until some jerk says something stupid to me.
There is often a complaint about too much magic in 5e, or about the ubiquity of magic in 5e; both points that I happen to agree with. But I think that this ubiquity is not just because of design decisions made, such as scaling attack cantrips, but reflect a fundamental design process. Simply put, 5e was designed with a spell-equivalency basis in mind.
While 5e is not perfectly balanced, it does try hard for balance. And one primary way that it is balanced is by using spells for everything. If you look at the design of almost everything in 5e- from monsters, to magic items, to classes, to subclasses, they are designed in terms of spells. Why have different rules for a magic item that allows you to fly when you can reference the spell? Why have different rules for a class ability that lets you teleport when you can reference a spell that lets you do that? Why have a special monster ability that lets it levitate when you can ... you get the idea.
Spells work. The spells have been tested. It is, for lack of a better word, easy. Do you want to create a new subclass quickly? Give it abilities that (with some "fluff") provide a spell equivalent, and an expanded spell list.
In other words, because this spell equivalency and use is so baked into the 5e system, it is likely to continue. Spells are one of the most basic building blocks of design in 5e.
Anyway, those are my thoughts on the subject. Curious to see what other people have to say.
But not just about psionics! Psionics can be a divisive topic; some people don't like psionics because they don't want "sci-fi" in their D&D (don't cross the streams!). Others don't like psionics because the rules have often been over-complicated or difficult to implement. Still others don't like psionics because they have often been added, or additional rules that come after the base rules (in an appendix, or in a separate guide) and it's never neatly integrated into the standard D&D adventuring model .... it's hard to account for the psionics when, for example, most published material isn't taking them into account.
Instead, I was marveling at how complicated and fiddly the AD&D psionics system was. And I contrasted that to the raging debates people were having over psionics in 5e; that the mystic or a dice pool was too complicated (for example) and that's why they went to a spell system.
...Complicated? The mystic and a dice pool was complicated? HA! Back in the day, we had to deal with casting times and weapon speed factors just to determine when your first level illusionist would die so you could sit in a corner and read Dragon Magazine the rest of the afternoon. Don't talk to me about complicated.
But no, that's not what I was thinking about. Instead, I was thinking about the difference in approach to design embodied by the two systems.
AD&D is a codification and expansion of OD&D; a set of rules that arose organically, and was customized to individual situations.
This is the bespoke suit. The suit is tailored to you, now. If you gain weight, lose weight, or someone else wants it, too bad. But boy, it looks sharp!
5e prefers simplified and universal resolution mechanics.
This is the leatherman multi-tool. It may never be the best tool for any particular job, but you know you can use it for most things.
I thought I'd briefly explore why I think that, and then have a conclusion explaining why I think this leads to the rampant "spell equivalency" design model in 5e.
A. It is better to look good than to feel good.
One of the most entertaining thing about getting five OD&D/AD&D players together, is that you'll get six opinions about how the rules work. Partly this is because of the amount of DIY as well as table and regional variance in interpreting and applying the rules. But partly it is because the rules and the rule subsystems are confusing and contradictory at times. The rules might say no resurrection for elves, because they have no souls. But the DMG might say, "Hey, the rod of resurrection works on elves." And if you want to know about how surprise works, well ... there's a page in the DMG that explains it, but it's wrong, and certain things have different rules maybe (Rangers and some monsters for example) and dex counts, except when it doesn't, and some monsters have dex (but most don't), and certain items and monsters determine surprise on different dice except that surprise is rolled on a d6 because it can effect the segment count in a combat and ... ahem.
The point is certain things can get complicated. And the reason for that is that AD&D (1e) essentially is a codification and expansion of OD&D and articles from The Strategic Review and Dragon Magazine. And as the game was growing, and situations came up, rules were added. A lot of the these rules made sense for the particular situation that they were being applied to! They were created because they "felt right," and in isolation, they do. If you need to create a new rule to handle, say, grappling and pummeling because you wanted some non-lethal bear hugs and hit, you just created a subsytem for it ... that involved percentiles, and math, and then you needed to break out the tables, wait, is guy wearing a helmet? Oh man ....
So there is a reason that this system is appealing; it grew organically. It often has satisfying solutions for individual rules issues (grappling being a notable exception). And it allows for a lot of flexibility in terms of abilities and differentiation; after all, if everything is custom, what does it matter if you add something new?
B. Teamwork means you always have someone to blame.
5e, on the other hand, is very much more modern in its design. Unlike 1e, you can be reasonably certain that most things can be resolved through the application of a d20 roll, instead of wondering if this will be a d6, d20, d00, or something else entirely. Attacks, saves, ability checks, and skill checks are standardized and roughly equivalent. While people can (and do) argue about rules interpretations, often because of issues related to either the use of natural language (various uses of 'melee attack' or 'attack' or 'unarmed strike' or 'melee weapon attack' etc.), or the intersection of "fluff" and "crunch" (how big of a mess does it make when a druid explodes after putting on metal armor).
But regardless of some complications (....hiding....stealth....), 5e is designed to be simple, robust, and flexible.
Side note- this isn't a Rod of Lordly Might measuring contest with other TTRPGs. It is entirely possible for other games to be more simple, more robust, or more flexible. We are just discussing D&D in this post.

This means that instead of creating multiple new and different subsystems to deal with variant issues, it is expected that you will use the systems provided within the world. You don't need a new subsystem for dealing with each and every animal you encounter; you simply apply the rules for "animal handling" with appropriate DCs and/or modifiers. And so on. Even when presenting a whole new section of rules (such as ships and water combat, as in Ghosts of Saltmarsh), the usual approach is simply to apply the current rules with a few extra guidelines (give ships initiative, give crews a quality score, and proceed on with the usual rules that tables are familiar with).
....and I would assert that this approach has an effect on the design of classes and player options as well.
C. My next trick will be to be a really happy guy. I'll just go around on the internet being really happy until some jerk says something stupid to me.
There is often a complaint about too much magic in 5e, or about the ubiquity of magic in 5e; both points that I happen to agree with. But I think that this ubiquity is not just because of design decisions made, such as scaling attack cantrips, but reflect a fundamental design process. Simply put, 5e was designed with a spell-equivalency basis in mind.
While 5e is not perfectly balanced, it does try hard for balance. And one primary way that it is balanced is by using spells for everything. If you look at the design of almost everything in 5e- from monsters, to magic items, to classes, to subclasses, they are designed in terms of spells. Why have different rules for a magic item that allows you to fly when you can reference the spell? Why have different rules for a class ability that lets you teleport when you can reference a spell that lets you do that? Why have a special monster ability that lets it levitate when you can ... you get the idea.
Spells work. The spells have been tested. It is, for lack of a better word, easy. Do you want to create a new subclass quickly? Give it abilities that (with some "fluff") provide a spell equivalent, and an expanded spell list.
In other words, because this spell equivalency and use is so baked into the 5e system, it is likely to continue. Spells are one of the most basic building blocks of design in 5e.
Anyway, those are my thoughts on the subject. Curious to see what other people have to say.