There are several MMORPGs which are supposed to come out in 2008. We have Age of Conan: Hyborian Adventures, Aion: Tower of Eternity, Warhammer Online: Age of Reckoning, and (maybe) Darkfall. There's also the expansion pack to World of Warcraft called "Wrath of the Lich King". Overall it looks like it's going to be a busy year for MMO gamers! If we speak about commercial success, I have no doubt that WoW's WotLK expansion will do really good, if not best out of these all. Then no doubt there's Warhammer Online with all the Warhammer fans coming to play; it also has a chance to cut into WoW's share of the pie, since the two are somewhat similar.
Aion, on the other hand, will be liked by Guild Wars and Lineage fans with all it's Asian eyecandy graphics and sexy characters. Plus, you can fly!
Darkfall is a big question; there are rumors floating around about it being released as soon as 2008, but many people believe that the whole game is "vaporware" and will never be released. It's really hard to guess since so little info is available about Darkfall, but if it ever comes out, it's going to be a really revolutionary MMO.
Last but not least is brutal and savage Age of Conan, which is going to be released in just a week as I'm writing this post. No doubt the game's brutal nature (gore, violence) and sexual content (nudity and implied sex) will attract some players, but what will keep them playing (or not) are the game features and the quality of gameplay.
The MMO market is constantly growing and evolving, and makes up a large part of PC games market overall. According to some statistics, there are over 16 million people worldwide with active paid subscriptions for one MMORPG or another.