I suppose they're more willing to experiment, really test their mechanics to the fullest. For example in the alpha build of Wwarcraft 3 they experiment with tripling the unit costs, leading to more tactful and more strategic games. Unit caps were almost ablolished. And with WoW of course they put their subscription money to good use updating servers, steadily increasing content.
I do like thier games. I sunk so much time into Diablo 2... I am a bit sceptical about Diablo 3 though, as it's not the same team that made the first two (as most of them left to make Flagship Studios, which recently fallen into a blackhole). I'm looking forward to it, but I'm not following it religiously.
The issue is with the MMO is that because there is so much stuff consumers will keep sinking money into it to get this stuff. Once they have realised they wasted their lives they'll dump more money into it so they can use this vast amount of stuff they now have, justifying their quest for digital booty. Vicious cycle.