1. The subscription cost. Simply put, many other games are just as good or better, and don't require a subscription. It's not worth it, especially considering all the extra things that have to be paid for when they should be included in the sub.
2. The community. It's full of whiny brats, immature trolls, and jackasses. I don't have even the slightest desire to speak to any of these kinds of people, but it's kind of required at times. But to be fair, it's like this everywhere. Other genres don't require quite so much player interaction, so they're better for me in that regard. But really, even though it's like this everywhere to some degree, this game still has one of the worst communities.
3. The auction house. I was going to say the dead server situation here, which leads to insane auction prices, but honestly I think I'd be annoyed by it even if I was on a healthy server. Player controlled economy just ticks me off, mostly because of the people who buy stuff out and sell it for higher prices instead of letting someone who actually needs it get it for the more fair price. I still don't understand how that isn't considered grieving, or exploiting, or ruining player experience, but whatever.
4. Class balance. There's not much to say here, it's a joke, and gets worse every expansion. I play to fight other players, and it's just not fun anymore. Healers are way too effective, there's too much crowd control, the ever changing balance that never really improves gets old, and so on. I'm a competitive player, and this just isn't the genre for competition.
5. The grinds. I'm sick of having to spend eternity leveling just to get to end game, I'm sick of having to spend eternity gearing up before I can actually have fun, and I'm sick of having to farm gold to afford the ridiculously priced gems and enchants each season.
6. The expansion trend. By this, I'm referring to the fact that I think every expansion has been inferior as an overall package to the one before it. It's hard to want to keep playing when every time things change in a major way, it's for the worse. And no, raid finder and things like that aren't the reason for me, I don't mind them.
7. Time spent playing. This one kind of blankets the rest, but it's still worth mentioning. I've played this game for many years now, and even if it was just as good as I once felt it was, I'd probably still be getting bored with it after this much time.