Guild Wars was quite awesome when it came out and I don't think you can underestimate the appeal of free online play... but I re-installed it last month and I was just amazed by how dated and uninteresting it seemed. GW is really MMORPG-Lite in so many ways. From character customization, to actual ability usage, to challenge. Really, it barely fits into the MMORPG genre to begin with in many ways since every single area outside of hub towns (which could basically just be called a lobby since they offer little of importance beyond a clear meeting place) is instanced. Instances are generally very small components in other MMORPGs.
That's a tangent, I apologize.
There's been a couple of threads on MMORPGs similar in nature to this recently. They mostly concerned themselves with upcoming MMORPGs and there's a few of those that look pretty interesting.
Currently, I've tried several on the market now. Several free and free beta ones have come and gone (particularly Korean ones), but I always find myself completely turned off by them within 15 minutes for one reason or another.
Guild Wars, as I mentioned early, is completely drained of its appeal for me.
Tabula Rasa was a good concept, but it hasn't been capitalized on yet. There's been some decent updates since its release, but it has a long, long way to go. I'd go so far as to call it a complete failure at this point. The world is uninteresting, there's little reason to want to advance. MMORPGs have to have a solid structure upon release. WoW had this, LotRO had this, Everquest had this, EVE Online had this and so on. Tabula Rasa really just did not and I don't see it recovering. The interesting combat components can only go so far.
World of Warcraft I don't play anymore. I keep trying to get into it and have been trying to since I bought the original version years ago. It's an excellent game and it's been tweaked so much since release that it's really pretty amazing. There's no shortage of people to group with, for obvious reasons (there's over 8,000,000 subscribers, a total anomaly in this genre).
My interest in it finally died somewhat recently. I was running around as a Blood Elf, trying to get 8 Lynx Pelts. Over two dozen dead lynxes later I had three. THREE. This had happened so much over the course of my playing this game over the years that I just decided enough was enough. Blizzard have become the kings of artificially extending the life of MMORPGs, something that seems completely unnecessary given the sheer amount of quests in the game and how many characters you can possibly create that can be almost completely different.
Dungeons and Dragons Online has some interesting concepts. Its menu systems are almost completely different from the traditional MMORPG designs. Unfortunately, not many people play it anymore... although its following seems devoted.
Personally I play Lord of the Rings Online. You can get the game for as little as $10 now, which helps immensely, obviously.I just think the game is fun, progression is rewarding and the story is integrated into the main game play in a way that's generally unheard of in MMORPGs. You don't have to be a Tolkien freak to get into it (I sure am not), which is also a plus. There's a thread here on this game if anyone has more specific questions about it.
The main thing I love about it is that the WoW-style 100 kills for 10 animal parts is almost completely nonexistent. Every so often I run into a quest where the drop is maybe 80% instead of 100%, but even that's barely comparable. If it seems bad, chances are you're in the wrong area.
There's trials of all of these (except maybe Tabula Rasa). I think the best thing to do is to find someone to play with in each of them and give them a try for a week. It's the only way to figure out which one suits you.