I was in the MMO market for quite a while, and at some point, I think I knew more about them, than most of you know about OC'ing. However I've been left behind a bit lately, as they had taken over my life, and I finally got the willpower to quit it altogether. I wouldn't advice you playing WoW. I know most people are praising it to the skies, but I really think, that it's destroyed the whole magic about MMO's. First of all, you can easily level up a character to max level in a matter of two weeks. In my universe, an MMORPG should be playable, with the same character for years, but in WoW people make new characters all the time, as they reach the maximum level. Also, no new features was implemented in WoW over older MMO's really, they just simplified the whole thing (this is a matter of your preferences - I'm just not very fond of it).
Now I think I'm done with my *****ing over WoW, and I can move on to what it's about. I've tried practicly all MMO's (except for the ones released within the least 2 months), and just before I quit the business altogether, I played the game Lineage 2. Lineage 2 has got a deserved bad reputation, as it was kinda bad in the old days. With the newest update (Chronicle 3), they moved the game to the next level though. It's very much like WoW, except that it has way better graphics (again a matter of preference), and actually has provided something new to the whole genre - the greatest PvP (Player versus Player), in the world. Before starting the game, I never really liked PvP in games, but in this game it's brilliant. There is PvP everywhere in the game, but it does have consequences. This adds for perfect gameplay, as the consequences is just enough to not making you attack everybody, but gives you a chance to do better roleplaying, and generally having more fun.
You also speak of RF Online - this is one of the few MMORPG's where my knowlegde is sparse, but I thinks it's very much like Lineage 2 (although I don't know how good the PvP is, as I haven't read up on it). If the PvP is equal to that of Lineage 2, I think you should give it a shot, when it comes out, as a new MMO is always better to start up in - it's really hard to do good, when the community is already tight and everybody has started out, like in Lineage 2... I love being there in the beginning at least
That's my 2 cents, I hope some of you actually read that, and didn't just think, that I'm an idiot
[edit:] Oh and Stevebaz talks about Warhammer Online - although it got cancelled, I think you should watch out on that one too.. I was so excited about it in the old days, I'm not sure if they changed the whole gameplay drammaticly now though.