You'll find lots of gamers who will sing the praises of the Basic Rules published in the early 80s (either 1981's Moldvay version or 1983's Mentzer version) -- and rightly so -- but it's rare to find many who express the same affection for the Expert Rules, whether the Cook/Marsh or Mentzer version. I personally find that a bit odd, because, for me, the Expert Rules are one of those places I can clearly point to and say, "This is what D&D is all about."
I'm certain that strikes at least a few people as odd. The Basic Rules introduce not only the rules of the game, but also its default setting: the dungeon. If anything is "what D&D is all about," it's the dungeon and surely the Basic Rules do a far better job of describing that environment and what it's like to play in it than do the Expert Rules, which muddy the waters with all this talk of wilderness adventuring and building strongholds and creating magic items and the like. Right?
Well, I'm one of those oddballs who takes seriously the notion that Dungeons & Dragons, despite its name, is actually about more than dungeon delving. After all, OD&D devotes a goodly amount of its sparse verbiage to adventuring in the wilderness -- so much so that the term "sandbox" is every bit as significant for old school play as is "megadungeon." Indeed, OD&D makes it pretty clear that, after a certain point, the focus of the game shifts away from the dungeon and toward establishing and maintaining a "barony." If you read reminiscences of the earliest campaigns in the hobby, such as Blackmoor and Greyhawk, you'll see that this was the case.
The Expert Rules present this shift in focus not as an "add-on" or accretion to the Basic Rules but as a natural development of them. Exploring and taming the wilderness, building a castle, and ruling a domain -- these aren't alien to D&D; they're a major part of what the game was intended to be about. This only makes sense, given the origins of the game in wargaming and yet they're topics that got short shrift even in AD&D, never mind later editions. In this sense, I'd say that, for all my issues with the presentation of Cook/Marsh and Mentzer, they're truer to OD&D than were their various descendants.
I can't stress this point enough, because I think it's a vital counter-balance to the tendency to see D&D, especially old school D&D, as solely about acquiring ever more power in the service of venality. Not only do I think that tendency does a disservice to D&D's origins, but I also think it exaggerates the themes of pulp fantasy to ludicrous heights. While not every Picaro will eventually settle down, many will, particularly if their players wish to continue playing that character beyond a certain point. The Expert Rules showed how to do that; they were where D&D's endgame was fleshed out and revealed it as the logical extension of all that had gone before.
I hesitate to say that the Expert Rules are where Dungeons & Dragons "grows up," because that implies a childishness to dungeon delving that I don't think is appropriate. Nevertheless, the Expert Rules are where D&D grapples with the nature of what it means to have reached high level -- to have "grown up" mechanically -- in a fantasy world. Certainly characters could continue to remain aloof from the world around them, remaining outsiders forever on the make, but how satisfying would that be for their players? The Expert Rules offer up new options of play, things that characters could take up in order both to expand the scope of the game and to ensure that beloved characters can continue to be played even after it no longer makes much sense for them to continue adventuring. These are the rules for King Conan of Aquilonia, as opposed to Conan the wandering Cimmerian.
I am ever more convinced that the progressive deformation of the original Gygaxo-Arnesonian vision of the game is due to the loss of D&D's logical endgame and its replacement by vapid alternatives. Only by restoring that endgame can Dungeons & Dragons again become the game it was meant to be.