I've played a couple MMOs in my day. I think the first one I tried was Martial Heroes, but I couldn't really get that into it. It's basically like any generic MMO, and set in ancient China.
Then I played Eudemons Online for a while, which is kinda like a cross between Diablo and Pokemon (hard to imagine, but if you played it you'd get what I mean). I got pretty frustrated with how the PvP system works, in that somebody one or two levels higher can kill you in one hit, made even worse by the fact that paying players had such a huge advantage that free gamers like me stood no chance in PvP and everywhere except the starting map and the insides of towns are freely open to PK (without warning, too, so you tend to get ambushed). Plus, there's a serious lack of quests and the game is in perpetual beta. Apparently development has been abandoned in favor of a newer project, which is really just the same game with a futuristic spin on it. The only really fun part of this game was raising and training your pet monsters.
I tried Talisman Online for a little while, but I couldn't really get that into that one either. One of my friends got me to play Runes of Magic with her a couple times, but once again it just didn't draw me in.
Atlantica Online has been my only MMO for a couple years now, and I play off-and-on. I like that there are a ton of quests, so it's not all that grind-heavy, and the cooperation is so much better than in the other games I've played. There are always people wanting to form parties or inviting you to guilds, and playing with other people is always more fun than playing alone. Additionally, the combat system is dramatically different (and better) than the other games, kind of Final Fantasy style. Instead of managing one character, you manage a group of up to nine (you start with about four and then get more as time goes by), and you can swap them out for new and better ones as you gain levels and complete quests. In this way, you can customize your group to extraordinary level (Want a group of nine spearmen? Not a good idea, but you can sure do it if you want!). The combat itself is turn-based but there's a 30-second time limit on each turn, so that keeps it moving fast. There are also PvP tournaments several times a day, where you're matched against players of a similar skill level (based on win-loss record). Paying players do have a pretty big advantage, but not enough to spoil the game, and you can beat players with higher levels and better equipment if you're good/lucky. Also, the auto-move and teleportation features make travel so much easier, especially since the world is really quite big. I've never tried running from Angkor Wat to Alcatraz, but I'm guessing it would most of an hour if you didn't teleport. The market system is the best I've experienced, and is driven by supply and demand because you put your items up for sale and you only receive money if/when another player buys it. The crafting system is quite nice, too. Basically, Atlantica is the only online game I've played that got just about everything right.
