Would you play WoW on a hard server?

I picked up the original Bioshock for cheap one day on Steam, and just started playing it this weekend. Being a slow, old codger, and not used to playing shooters, I can still play that game perfectly well by choosing the easy difficulty setting. Meanwhile younger, and faster shooter veterans play the game on medium or hard. Why does everybody assume that this wouldn't work for MMORPGs?

With so many people complaining that "WoW is too easy", there should be a market for having a few "hard" servers, where all mobs have twice the health, and hit harder, while the players have exactly the same stats as normally. On a Bioshock forum the veterans discussing the game are "of course" playing it on hard. Why does everybody assume that for a MMORPG everybody would play on easy, while continuing to complain about lack of challenge? Maybe it is just a historical fluke, that the first MMORPGs didn't have difficulty levels, and now everybody assumes they wouldn't work, just because nobody ever tried them.

What about you? Would you play World of Warcraft on a hard server? If not, why? And what do think is the difference to a single-player game, where difficulty levels work perfectly well?