Spells, man, spells. The downside of spells is that characters who know spells know a lot of them. Your first level Wizard and Cleric both have over a half dozen spells prepared, including cantrips, and the Cleric can choose from a much larger list every time they go to sleep. So unless you plan to memorize the details for dozens of spells, you need either cards or a book in front of you to play a caster. Or worse as a DM who has to run a caster.
On the other hand, spells are very discrete in terms of their function and are easy to look up — they're presented alphabetically. And you know which ones you use, so you can buy the cards or write them down.
Once you remove spells, and assume that your players have done their job and written their class features, race features and feat features out, there is not much you need to know to adjudicate all actions in D&D. The general-purpose rules content is pretty tight. You need to get the conditions down, but I find the to be very intuitive and haven't looked them up in ages. (I plan to cover the conditions up on my DM screen with information I don't know.) There's very little of the "If this thing happens then ..." style rules, which some other games are full of, and necessitate trips to the Index, because which chapter covered the rule for fighting while lit on fire? Was it the Combat chapter? Was it the Skills chapter (under the Firefighting skill, which allows you to add half of your skill bonus to mitigate the penalties imposed by the Lit on Fire condition)? Or was it under the Adventuring chapter under "Adverse and Harsh conditions?"
Yeah, that's a major component - but what's key about 5e here (and most editions of D&D and D&D-adjacent games) is that each individual spell operates as a keyword which can get re-used by monsters, magic items, NPCs, etc. That's what I said upthread - the text of Fireball is as much a rule as, say, the rules for Cover, because it will be referenced by other game content.
You don't get a Staff of Fireballs which can hurl X number of 20' diameter orbs, each doing Nd6 damage; you get a Staff which has charges with which you cast Fireball at various levels. A Gold Dragon doesn't know the following list of tricks with all the game text; she knows all of the following
spells and can cast this number of them. And so on.
Oh, and since Storyteller came up, upthread...
Here's my favorite possibly-apocryphal Greg Stolze story, re: White Wolf and Game design.
[sblock]Older White Wolf games have three ways to scale difficulty, IIRC - you change the target number you need for successes on your d10's, or you change the number of d10's you're rolling in your pool, or you set the number of successes needed as something higher than one. So there's three axes along which you can adjust difficulties - or maybe it was just two of those? Hell, it's been forever since I played a WW game.
Anyway, when Greg Stolze went to work for them, he asked the designers how they decided which of these options to take, and what the various options meant for the mathematical chances of success on any given task. This is a completely obvious question that any RPG designer nowadays would expect, and should readily answer. But
none of them had any idea - they just kind of operated by
feel, and didn't see anything wrong with this. They knew what made things harder or easier, but not really by how much.
So if all of this is accurate, the Storyteller attitude of being above "roll-playing" went all the way up to the top, where the authors didn't even think the math underlying their dice rolls was important enough to figure out.
And that's how the One Roll Engine (the system Stolze used in REIGN) was born.[/sblock]
