I used to play WoW quite obsessively, but it was for the gameplay and to be with my friends who got me into the game.
I quit, and only looked down on that game itself, not MMOs in general. I've tried to find good MMORPGs, but most of them are very very very similar to WoW in combat and experience.
The only true MMOFPS/RPG I've seen/played is Planetside, and while it was very good, it would have been better if it didn't have a dated combat system. It should have not been advertised as an FPS because it was much more strategy-involving. But it is very fun for the same reasons as WoW - not the combat, but the planning, the execution, working together, etc.
The other problem with it was that it was PvP only, which gives it the staying power of most any online FPS - can't play it forever, even if it's constantly updated and support (Which it wasn't.)
I've never played an MMORTS, but they look good, especially BattleForge, and I've considered them. And if you think about it, isn't Starcraft a MMORTS?
MMOSHMUP, I'm waiting for Valkryie Sky.
MMOFPS, now, like I said...I think Planetside had good ideas, and bad implementation. If they made a new Tribes that worked like this, it would sweep away the market.
But anyhow, I think Quake Live is basically what a MMO Arena FPS should look like.
I have high hopes for Global Agenda, Huxley, and a couple others, but...the market will most likely be over-saturated and all will fail.
MMORPGs can be wonderful - they just sacrifice gameplay to get accessibility. I think a Fallout MMORPG could be fantastic, but I get the feeling that the world will be too static and having other people will ruin it. Fallout will simply not be Fallout with many players.
A KOTOR MMORPG could be wonderful, but again, they might just take what makes it KOTOR out of the equation. Which is the exact problems with MMOs. In their current form, they just aren't fun.
I have high standards of gameplay. And the things I want in an RPG are not currently in an RPG. I guess City of Heroes gives me some character customization, but as for changing the world around me? I don't think any have that, or if they do, it's in some controlled state. I can't create a city. I can't destroy a city. If I can put down a space station, it's only viable and useful to do so in a certain area. Etc, etc.
Anyhow, to wrap this up, MMOs can be good, they just, aren't good. Some games like Pokemon or Mechwarrior, they seem DESIGNED to be an MMO one day. Some games (Fallout, Mirror's Edge) just don't seem that they would work like that though.