I played for about a month after release and loved it despite the bugs... I got tired of running around completely alone, though. If the game was more populated, I'd go back in an instant.
In my opinion, at least, the reason that WoW has done so well is not because anything in it is exceptional, but because it doesn't have many major flaws. As others have said, it runs well on most computers, caters to both casual and hardcore gamer…