Old MMOs are going to have a natural advantage, because once you've hit the cap and start getting bored, but you don't want to quit the game, you help other people, or you make alts and learn to enjoy the game more the second time around (usually by taking your time and being more generous). Also, most of the gameplay is cooperative, so you don't talk to people you're actively competing against.
In other words, the politeness in MMOs compared to other games is a natural growth of the gameplay. I wouldn't say WOW's community is significantly better than any other MMO I've played. (Maybe Ultima Online...)
For really interesting multiplayer experiences with communities which could have gone either way, I nominate Demon's Souls. The game actually encourages jumping into someone else's game to player kill them, but it does it in a structured way that makes sure the host player always has a natural advantage. Maybe it's this structure, or maybe it's because you can't communicate verbally (only through gestures,) but even the black phantoms (invading players who are trying to kill you) are extremely courteous. Many will bow, leaving themselves open, before they attack you. Some even jump into your world and then help you kill monsters and clear the level instead of attacking you, which gets them nothing for their time. Kind of like high level players helping others in MMOs, I suppose -- they just want to spend more time in the game world, and helping others is one good way to do that.