Two things: level progression that allowed us to assume that you'd age about a year per level, and the simplicity that enabled every single gamer I knew to try out his hand at homebrew DMing! Not everybody found they liked it, but if you had the DMG, PHB, and MM in 1979, you gave it a shot at least once and it didn't take that long. Yes, a lot of what they produced was crap, but Sturgeon's Law applies. You could write up an adventure, stat a few NPCs and monsters, and success/failure depended on the people involved - your skill as DM, their skill as players - and you didn't have to worry about the feats and the skill points and so on. And yet, there were enough rules in the book and floating around the subculture that you could mix and match to your comfort point of complexity. Want a hit location chart, weapon speed, a magic point system, critical hits and fumbles? Use 'em. Don't want 'em? It's your game, man. Can you imagine if I decided I didn't like feats and outlawed them in my campaign? (I don't like feats, actually.)
Mind you, I used to find myself intensely frustrated trying to simulate people who had lives other than finding monsters and taking their stuff. The secondary skill system didn't cut it and we were always making house rules to cover things like, if you weren't a ranger, could you still find food in the wilderness? Could you make your own weapons? And although I am of the opinion that the present rules for crafting magic items stink on many levels, at least they exist.
I can't do homebrew these days. I have to use the modules, and all modules offend our communal playstyle to a greater or lesser degree and have to be modified extensively and it's all so much work. One of the other players who has shown considerable talent for spinning game elements into campaign depth has said casually that if he ever DMed it woudl have to be AD&D, and I for one wish he would.
Actually I wish anybody would DM any different system once in awhile, just for a change. No system is worth playing 100% of the time.
I miss activity cycles, knowing what was active when. I miss morale checks. I miss the treasure tables and standard xp rates and pregenerated random monster checks. My players calculate xp for me - I simply can't grasp it, somehow.
I also miss the stamina that let me stay up till three in the morning, accumulating character sheets as other players fell asleep on the floor, until only the DM, the guy with the 16th level monk, and I are left to play out the fight against the gorgon. But this is balanced by gaming with people who have outgrown the old-fashioned dungeoncrawl that made no freaking sense.