I love this idea from Matt on WoW LJ -- he points out a few great examples of what he calls "incidental RP." The vast majority of players in the game wouldn't call themselves "roleplayers" (and lots of them might make fun of people who do), but all of us, in playing the game, are buying into the lore and the roles we're playing to a certain extent. If you've ever yelled "for the Horde!" or felt a strange hatred towards Gnomes, guess what -- you've been roleplaying.
His examples are a little more silly -- his girlfriend hates parrots (because one of them once killed her in Deadmines), and he'll sometimes answer NPCs, even though they never respond. I do the same thing -- /salute an NPC after grabbing a quest, or yelling "and stay down!" after dropping a major boss or a mob that's given me trouble. That kind of stuff is a far cry from the heavy RP that can go on in game, but just the same, it makes the game more fun. And just like good art, it puts you even more in someone else's (fictional or otherwise) shoes -- what would it feel like to be someone given a quest, or a real member of a group like the Horde? Maybe that's why roleplaying is the wave of the future.
Any other great examples of incidental roleplaying? It's another mark of how great Blizzard's game is that the setting, art, and experience of playing pushes people who wouldn't normally get into character to really think about what the person they're playing is all about.