I'll just post some nonsense on some of the ones I've played.
Ultima Online:I've played UO since 1998. Sure you start out frustrated but by the time you give it a chance you have a lot of reasons to play. It was my first MMORPG and the "newness" is probably what got me hooked at first but now that all this time and effort has been put into it, I just can't stop. UO provides customizability unlike any other MMORPG. It also does things differently than any other MMORPG. The skill system is unique and enjoyable instead of just leveling. The game now does have a "safe" facet which bugs me personally. I tend to do all my stuff on Fellucca. The fact that you can just be a profession (lumberjack, miner, blacksmith, bowyer, cook and so on) instead of your run of the mill "hero" type is pretty cool. The player run events are excellent.
Dark Ages:This is one I played since "DEOCH 11"
I quit playing it but what I liked about it was the way the world was run. Player guards, player judges and so on. The community was excellent when it first started. Since then it has become overrun with morons.
Guild Wars: Been playing for a little less than a month now. It's pretty cool. I like the change of pace from regular MMORPGs since it really isn't one. The game is enjoyable so far.
Asheron's Call: Highly underrated. This game was awesome. Housing was available with the first expansion which added to the immersion. I only played on Dark Tide (think that was the name) the pvp server and if you thought UO was bad in terms of being slaughtered at any given point, this one was worse. I enjoyed that about it though. The fear kept it fun. The gameplay was more action oriented than anything out at the time which was excellent. You could actually dodge stuff and so on. I quit playing but have very fond memories of this one.
Asheron's Call 2: They really disappointed me here. They forgot what made the first one fun. It is now no longer in existence.
Rubies of Eventide: This one went under quick. I remember beta testing the original build. I actually liked that better than when they turned it into a standard MMORPG. Nothing great here.
DAoC: The RvR in this is the main grab and it is excellent. I haven't played since probably 2003 but really enjoyed my time in the game. The whole concept is pretty cool. I played a Hibernia and Midgard character. Good balance and a fun enviroment.
Meridian 59: The old school. Started it all. Never really got into the game because of the weird pay setup. They have since changed that and even upped the graphics a bit. If you want to get into a first person (daggerfall like) mmorpg, this is it.
Everquest: Played this for a while but was too into UO at the time. It consumed one of my friends which is pretty much why I despise the game but it did have a grand vision. Only WoW has surpassed the crazy fan base that this game had.
World of Warcraft: Quit playing this but it is a very good game. It doesn't really do anything special but it does a lot of stuff really well. Great execution.
Mimesis: Not really worth it. The game looks pretty good and I enjoyed the setting but the engine just sucked.
There: Like Second Life but I enjoy this one more. I have a lifetime account from beta testing it. The customizability is very cool.
Underlight: Very similar to Meridian 59 in it's setup but a sci fi type setting. Interesting but not very good.
Matrix Online: Unfortunately this one turned out to be really boring. I enjoyed the setting but didn't enjoy the game.
Shattered Galaxy: From the same creators of KotW and DA. This one was pretty interesting. Futuristic setting and the ship battles were pretty cool. Nothing that really kept me playing but it was a interesting beta test.
EQOA: EQ on the PS2. This actually was quite a bit different than EQ in its engine. The game was more action based. I enjoyed it til about level 30 then just quit playing. Fun while it lasted and better than FFXI
FFXI: The world was cool looking but the game itself was slow paced and pretty boring. The only real thing I liked about it was that you could try each job with one character.
That's all I can think of right now.