I've been ruminating on this topic for several days now. It was easy to list dislikes right off the top of my head, but listing why I still LIKE it (to the extent that I have, at times, been more interested in a 1E AD&D game than another 3E game) has been much more difficult to nail down. So that very thing goes to the top of the list.
There is something UNDEFINEABLE that I like about. Maybe it's got a lot to do with nostalgia. A significant part, but that IS only part of it at best. I miss that idea, only fleetingly suggested in places, that this was ultimately loose tenets for organized fun, and not cold, calculated rules for a GAME. "Game" in this context having connotations of competition, perhaps not necessarily between player and DM but between player and player. I see that in the notion of finding fun not in the PLAYING of characters, but equally or moreso in the mere planning and creation of characters through the complex coordination and application of ever-larger and less manageable pools of components. Win by character creation.
Were some of the old rules complicated? Sure. Yet I find myself MISSING some of those needless complications and for a couple of years now having NO interest in the expanding complexity of 3rd Edition sourcebooks. I'd almost rather go back to a flawed, glorious mess of fun that is 1E and try to fix what's wrong with it than start with a crisp, cleanly designed system that is 3.5 and try to find for it the soul it seems to have eliminated.
What I like about 1E AD&D is that it is not a game of rules, but just rules for a game. Truly, my penchant for 1E boils down to that, and not some list of rules crunch or flavor text. It is the largely unwritten philosophy that lies behind the current game that makes the complete absence of philosophy behind the old game likeable.