Here's a list of the MMOs I've tried and why WoW is best.
EVE: It is a job. I know a lot of people will tell you otherwise, and maybe the two weeks I spent playing it were just the "lead-up" to the actual fun, but it really was boring to me. Nothing at all happened, really... except for the douchebag PvP pricks who will take every opportunity to ruin you. Why? No, they're not "pirates", they're not from a rival faction, they're just assholes who like to blow up people weaker than them. Normally I'd be okay with this (see WoW PVE vs. WoW PVP), but there is simply no option to play through by yourself. Woo.
Tabula Rasa: lol. I was in the beta, I registered for the actual retail product, and it was... well, it was okay. One thing really hung me up on this game though... maybe they've changed it now, but back when I played there were several trees you could progress upon to specialize your character. IE - Soldier to Commando to Sniper or something like that. Once you hit level 30 (I think) and choose your last specialization, it is the most immensely dull game I've ever played. Plus combat is severely broken.
Age of Conan: It is quite possible that since I cancelled my subscription last year, they have fixed it. When I played, it was a mediocre WoW clone, with improper or misleading tooltips, severely unbalanced classes, a very vast sense of "what in the fuck do I do now", and a general feeling of "this isn't even done". It did have admittedly amazing graphics. Also, boobs. As far as the eye could see.
Guild Wars: It was alright, but again, once you hit the final level and got your final skill it was basically just a competition to see if you could squeeze out just another one damage per second so you could win in arenas. And let's not even get into the userbase... an MMO with no monthly charges = fifty guilds called "COCK" or "BUTTS", and everything you see in city chat is along the lines of "omg u r a fag lol". I understand this is, after all, the internet, but jesus christ man... I play games to get away from pricks like that.
Star Wars Galaxies: Just wait for The Old Republic, unless you're really keen on emulating the old Galaxies. The way it is now is hilariously terrible.
The Matrix Online: It's defunct now, so I don't even know why I bring it up, but this is another classic example of SOE taking something good, turning it over, and giving it the gears Deliverance-style until it's no longer playable.
Everquest: I didn't play it nearly enough to get a great idea of what it's all about, but I will say that I had a pretty okay time with it.
Everquest 2: Looks good. That's about it.
Darkfall: Commonly called "Darkfail" by yours truly, and for good reason.
All of those Korean MMOs: All of them are universally bad. Unless you've got a no-limit credit card.
Addendum: Actually, not all Korean MMOs are bad. Shot-Online (a golf MMO) was actually hella awesome until the pay-to-play elements were added.
Why do I think WoW is great? Simple. It does the MMO thing the best. It sets the bar for every other MMO. Leveling up isn't necessarily fun, but it's not the chore most people ***** about, and there's always something to do.
A few of the biggest things I like about WoW are thus:
-A lot of people play it, so finding a group isn't always hard and there are some really cool people on there*
-The graphics... even though a lot of people ***** that it's too "cartoony", I love playing a game that isn't all grimdark like every single video game that came out this year (browns and greys, woo)
-The entire game feels polished. Everything has been gone over many times with a fine-tooth comb to fix bugs, to make sure tooltips are actually helpful, to make sure that you can have fun. I have never played an MMO where everything works before I played WoW.
-The monthly fee weeds out a lot of people who simply don't belong in contact with other people
-It is still easy to be a jerk as a Rogue
-And finally... it's just plain fun.
The cons of WoW, as I see them, are:
-There are still a lot of people who are so retarded you wonder how they are able to use a computer
-Since everybody already has a level 80 character, finding people to level up with is a little harder than I remember it (I used to play pre-BC)
-The expansions contain everything fun (flying mounts, new zones, new equipment)
-The expansions are also a little pricey: I paid $40 CDN for Burning Crusade and $60 CDN for Wrath of the Lich King
-It is still easy to be a jerk as a Rogue
I dunno. It's not a terribly big decision, but for what it's worth I love WoW and think you should start playing it.