2e's ability to fairly easily swipe anything from previous versions of the game without much fuss was certainly peak. It's sad that's been lost, but I'm not sure how you'd maintain that (do I dare say it? I do!) "backwards compatibility" without keeping the game from being able to evolve.*
*Like, I know that there are people in this very thread who didn't want the game to evolve, but I'm sure everyone can agree that there was stuff that needed to be improved upon that would eventually orphan older concepts. I'm not, of course, saying, that everyone agrees on what that stuff is, lol.
A lot of the anecdotes shared about D&D in the pre-internet days that I really enjoy is how everyone was playing their own version of the game, despite the conflicting statements Gary makes about that in the DMG; ie, "it's your game" and "if you're not playing the game as written, you're not playing D&D".
I know in my group we had the following rules that sort of manifested out of the ether (includes 2e stuff):
*4d6 drop 1, re-roll all 1's somehow became the standard for rolling characters. At some point, scores below 6 became obnoxious to both DM and player alike (we had a player who decided the only way to roleplay low Int/Wis was by being completely obnoxious).
*A natural 20 doubled all damage from an attack. A natural 19 increased damage by 2.
*A natural 1 on an ability/NWP check was considered "masterful". One DM would always describe works of art and such as "obviously he rolled a 1 to craft it". Similarly, it somehow got decided that, regardless of your actual check, actions performed by characters with extra slots devoted to NWP's were "better" somehow. I remember being annoyed by this- there was a drinking contest, and my character with an 18 Con and Endurance made his check against an NPC with a lower Con but 2 extra slots devoted to Endurance, so the DM said he won(?!).
*Comeliness was an ability score. The stated benefits of said ability score, as far as I could determine, were never used. It was just shorthand for how hot you were. Naturally, this led to (many) NPC's with ridiculous high Comeliness. The idea that negative Comeliness was attractive to evil people and vice versa was flatly ignored.
*Comeliness could be lost when "it made sense". One guy took some fire damage rescuing an NPC out of a burning building. Despite not actually taking a lot of damage, they gained "horrible burns" that cost them 6 points of Comeliness. I felt this was mean spirited, but everyone else in the group seemed to think it was perfectly fine.
*Charisma, as an ability score, was basically used for checks. Not once did I ever witness a DM rolling for NPC reactions. Few of the players wanted bases and followers for whatever reason. # of Henchman didn't really matter much- the only way anyone ever got a permanent Henchman was the Knight card, lol.
*Spell effects from Wild Surges ignored Magic Resistance. Can't explain how this came to be.
*Magic armor granting bonuses to saving throws was generally ignored. If a player brought it up, the DM would allow it, but only grudgingly. I guess this isn't really a houserule, and more of a "yes, it's in the book, but rather than ban it, we just glare at people who use it".
*One that might be familiar to a lot of players: magical auras that blind/stun people who dare to cast Detect Magic because of how "awesome" they were. Similar things could happen with really Good/Evil individuals. Paladins would find themselves having to make Con checks to not throw up around really Evil people.
*Paladins again- Detect Evil wasn't a spell, it was just on. You knew if you were around Evil people 24-7.
*Rangers could be Evil. Again, no idea how this got started. Also, Rangers had to be racist, and Favored Enemy would apply to related creatures. For example, there was a Ranger player who wanted Favored Enemy for Duergar. The DM ruled they hated ALL Dwarves.
*Full Plate and Field Plate reduced damage from falls. I have no idea how this got started.
*Merchants always rounded costs up to the nearest gold piece, and wouldn't accept copper. Mountains of copper coins still exist out there in monster lairs, untouched.
*Fighting large foes always resulted in a chance for the enemy to fall on you for damage. I've even seen characters die from this!
*Every NPC had character class levels (I blame Forgotten Realms for this) yet somehow, only the PC's could solve problems. I don't know if this counts as a "rule" either, but there it is.
*Bow specialists could use their weapons in melee combat. I believe this was a 1e Ranger ability that got ported over.
*Demihuman level limits: this was a strange one. We recognized that they existed but rather than abolish them, we went through strange gyrations to try and ignore them. Using the expanded limits in the DMG (having high stats let you rise to a higher level), requiring double xp to level up beyond your limit, single-classed characters had their cap raised by 2 (this might be in the DMG, I can't recall).
One day, we just stopped caring.
*Anyone can multiclass or dual-class. This came out of things like the Conan adventures, Lankhmar, and a section in the Complete Book of Elves. Basically, if you showed up at the table as a Whatever/Whatever, it was just accepted. If someone said "hey, you can't do that", the reply was generally "so?". Also, one player was allowed to multiclass as a 1e Ranger/2e Ranger. With a Kit. Amusingly, that same player made a fuss about a multiclassed Bard/Thief (actually legal according to the Bard's Handbook) because you can't be a "Rogue/Rogue, the PHB says so". Ranger/Ranger apparently is ok because the 1e Ranger isn't a "Warrior". Insert confused Jackie Chan face here.
*Starting age. This turned into a minor war when I made a Wizard in his 20's. "Wizards have to be old! You should roll for your age like the book says!" (despite nobody ever doing that). I was forced to produce multiple pictures in D&D books of not old Wizards (including a bevy of young supermodel Wizardesses, lol) and even a few NPC's, like a 10 year old 1st level Wizard in one of the adventures in Wildspace (The Sacred Firefall, as I recall).
Once my character was approved, we then had a run of people wanting to play super old casters to get the bonuses ("Well, if you can be whatever age you want, then..."). Especially Elves. Nothing like a Venerable Grey Elf with Int 21 who will still be alive and kicking for another couple of centuries, lol.
When a DM "decided" he had to make a System Shock to avoid a cardiac arrest after coming face to face with a Death Knight and he died, oddly nobody else was upset.