WoW is my first and only MMORPG so far.
I agree with some of the faults people have mentioned:
Lots of the game is just sheer repetitive killing, or "go fetch" quests. Of course, you could say the same thing about many single-player games too.
Some of the quests are pretty mundane, and because of the game structure, you DO end up waiting for monsters to respawn.
There's a definite lack of cohesive plot, compared to single-player RPG's. In fact, that's one of the things that bothers me most about WoW. Almost nothing you do in the game has any kind of permanent effect on the game-world. There's no sinister monster planning your demise in the background like in Baldur's Gate. You don't get to see an entire city destroyed (and permanently gone) like in Tales of Symphonia. There is no ultimate bad guy that you have to kill. And even if you do kill somebody, he'll be back in 10 minutes. I guess there are some big world events, like the opening of AQ, but I wasn't a high-enough level when that happened to take part in it.
So why do I keep playing?
I spent lots of time leveling up my primary character to 60 (the maximum level), playing for long long hours. During that time I explored new areas, found new items, and met lots and lots of other players.
Much of the appeal of the game for me is just seeing new things. I enjoy seeing new creatures, a zillion types and combinations of armor and weapons, bizarre pets, and strange landscapes. The first time I saw Ironforge, my jaw dropped. It looked like a dwarven city straight out of Lord of the Rings.
I love Ungoro Crater (complete with dinosaurs), still get creeped out by Duskwood (and all the undead), and hang out in sunny Arathi Highlands when it's raining outside in the real world.
As a single-player game, WoW isn't that great. As an interactive multi-player game (both cooperative and adversarial), it's fun. Grouping with other characters and utilizing different class's strengths to create a synergistic killing machine is fun! I joined a good guild, and enjoy playing with other people.
That being said, I've reached the point (as a level 60) where I'm not finding the big group raids to be much fun. I spend 4 hours in Zul Gurub grouped with 19 other people (and it takes an hour to get enough people in the right class combination to get together in the first place), to die several times, and to maybe kill a big boss monster, so that we can split a paltry 1 or 2 epic items. I much prefer the instances where you only need 5 people, so you can form groups much more quickly, and have a greater chance of getting some good items.