Well, at least the original edition was not a game about role-playing - any more than Clue is.
On the other hand, we have all known people at the D&D table who just didn't go in for theatrics, accents, or back story. They played their characters as well-balanced, flexible, unremarkable protagonists. I have no doubt that they enjoyed the danger, exploration, problem solving and treasure finding of the game as much as the others. They may have even been playing closer to the pulp-adventure model than the more characterized players. More often than not, the hero of a 1930's adventure or sci-fi tale was a rough block of a character with an Anglo-Saxon name and few personal characteristics. It's a technique whose visual equivalent has been described in comics by Scott McCloud: through the abstracted narrator or protagonist, the reader gains a greater personal immersion in the fictional world.
There are few if any rules in the original D&D game that direct, enforce or reward role-playing. A Lawful alignment was the player's to lose, giving only social benefits. It's anyone's guess, for example, whether a Lawful cleric who acted evilly would lose their powers or just turn into a Chaotic one. Otherwise, characterization was left up to the players and, of course, the referee as master NPC roleplayer.
It's AD&D that started holding mechanical game benefits hostage to role-playing. The most notorious case was the paladin, but also rangers and druids, assassins, and clerics (with the authority given to the DM to skimp or change spells for bad alignment playing) had to watch how they acted. The vaguely defined moral universe of the alignment system created a culture of pettifogging casuistry, where "do baby orcs have souls" and other choice questions were debated throughout the rec-room seminaries of the land.
While everyone accepts this as part of the AD&D games and smirks nostalgically at alignment shenanigans, I'd argue that this is not the most shining moment of the system. A paladin bound by twelve clearly defined behavioral commandments similar to the one about accumulating treasure, for example, would work a lot better in play. Cut loose from the alignment system, that class would have the additional interest value of varying just how far you can get from Lawful Good and still play within the rules.
Of course, there is a side branch of development toward games with true role-playing mechanics, whether built into the system as with Pendragon, or included as options, as with the personality flaws in GURPS. Other systems on the indie/experimental side pump the importance of role-playing even more. And many DMs in D&D, too, have had the practice of rewarding in-character play with experience points.
I personally don't see XP for role-playing as necessary to the D&D game. Role-playing is something that should be its own reward because it's fun. Many players take to it even if only on a basic level (grunting fighter, flighty elf). If done well, the whole table benefits as the audience. But what I don't want is having to hand out XP that encourage players to do bad roleplaying (although even that can be entertaining in its own way); that pay off on prima-donna character motivations that work against the party in a game best played cooperatively; or that make people who aren't really cut out for play-acting feel left out. Nor do I want the spontaneity of the role-playing to fall victim to the overjustification effect. "Woops, session's closing soon, better get in my stock catch phrase so I can cash my XP check."
Now, I'm aware the "controversial" D&D != RPG statement has been made many times before. What I want to do here is make it a positive, instead of a slam on a game seen as too combat oriented. As with the other player-skill aspects of old school gaming, just because there aren't rules for roleplaying in D&D doesn't mean it won't happen. The good thing about D&D, done right, is that there's always a safe place at the table for the person who would rather just play an adventure game.