Tobold linked to a new blog about MMO gaming called Hardcore Casual, and the first entry is a nice one-- it's all about the differences between a "carebear" game like WoW, and the much more hardcore games of the past. His big example is Ultima Online, where players could gank each other and actually loot the corpses. Playing a game where others could steal your armor is very different from playing a game where BoE means it's yours forever.
But you don't have to go back that far to find a really hardcore game-- in Blizzard's own Diablo 2, you could play "hardcore mode," which meant that when your character died, that was it. Game over, no respawns, nothing. Either you lived and beat the game, or you died and lost the ability to play your character and everything with it.
To tell the truth, I'd love to see a server like that in WoW. Not because I'd love to play it (what are you, crazy?), but because the stories coming off of that server would be terrific-- we could all just stand in awe of the character who made it to 70 on the Hardcore server. Or would that even be possible? Especially if the server was PvP (and it would be, wouldn't it?), it seems like an impossible challenge to keep a character alive that long, especially since the other side would be gunning for anyone who got too high. Would you play on a hardcore server, where you could lose your life (and/or your gear)?