Wow...where to begin? Well, coming from BECM, 1e and 2e predominantly...the stuff that immediately springs to mind (I'm sure there's more, but the finer details are long forgotten):
1. PCs begin first level with automatic max HP. But only 1st level. Re-rolls for results of 1 or 2 at subsequent levels were a matter of course if not an official "rule change."
2. Encumbrance, after a time/when I was in a fairly experienced group, was basically ignored.
3. Ammo tracking was basically...overlooked...until it reached ridiculous proportions or we were in a "serious" big fight. "You guys haven't been in a settled community in six months...[to the archer ranger guy] how many arrows do you think you have left?" In one particular "Underdark" campaign, as I recall, this became a real issue and we had to reinstate "arrow counting." Sling stones was never a question...just assumed the any sling-user was occasionally noticing/picking up decent pebbles along the way.
4. At some point in 1e's reign, well before 2e, Racial level restrictions were out the door before it was even opened.
5. Weapons reach, speed and the Weapon types v. armor tables were ignored/not in play. Roll to hit. Add in your ability & magic bonuses as applicable. Roll damage if necessary. The extra levels of +/- weren't really needed or desired.
6. Spontaneous spell choosing (from spells you had in your spellbook) for arcane casters at 2 spell levels below your highest castable spell level: a 5th level mage gains access to 3rd level spells. You still have 4 first level spells [slots] available for the day, but you can choose them, as needed, instead of having to memorize at the beginning of the day. At 7th level, the MU gains 4th level spells, so 1st and 2nd level spells can be chosen as needed. This was to help put them en par with clerics who (until the clerical spells "Spheres of Influence" of 2e) could just pick any spells they wanted.
7. Following that, though I honestly don't recall when/how long, MUs could apply their Int. modifier to spells per day, the same as Clerics and Druids could with their Wisdom bonuses.
--7a. Shortly thereafter, the "spell tier" tracking system was instilled and instead of a 17 Wis. cleric/17 Int. mage getting auto-: 2 1st/2 2nd/1 3rd (or whatever it was in those days), we just said: "Your ability bonus is +3, you get 3 tier of spells extra per day. Whether you chose to use that for 3 extra 1st level spells, 1 1st and 1 2nd or 1 3rd level [tier] spell, each day, was up to the player.
8. Racial class restrictions were basically dashed, within the AD&D system anyway (race as class in B-X/BECM was fairly well respected), after the advent of Unearthed Arcana.
9. Also after 1e's UA, and particularly after Dragonlance's books and modules (with the introduction of "Draconians"), and "Spelljammer" (though I think we played ONE session of it), custom classes and monsters were all the rage for a while. Things pulled out of (or based on) stuff from TV, cartoons, comic books, other game systems, were all the rage there for a while. Let me see it...as long as it wasn't overpowered [we had no concern or conception of something called "Balance" then. You could be more powerful than other PCs, do/have abilities they didn't, but don't get too crazy/out of control], we had some kooky, but fun, creations that were, most assuredly, not in line with RAW.
On the flip side, one thing we never really questioned or had any arguments about was the idea of changing or eliminating alignment restrictions. Everything "made sense" as far as we were concerned and alignment simply stayed in play, as written, without problem.
