The in-game music. I even usually loop it. I know not everyone likes to do this, but I can listen to any other music any other time. Listening to the game music as I'm playing keeps me more immersed in the world. It also doesn't hurt to know how much work and passion went into creating the game music by the composers to make you appreciate it even more.
I bet you are right here at least in terms of the passion to create. Sadly, I've never actually heard the music in WoW, it's the first thing I turn off for any game I play... too many games have awful music. Maybe I should try it someday.
You should definitely give it a listen at least once. It may surprise you.
WoW music is probably the least enjoyable MMO music on the market. I listen to Pandora when I play. I test the in-game music for a month or so with each new expansion, notice it's still rubbish and turn it off. For a company that makes million they could stand to shell out a little more money on decent composers.
I get that you dislike the music on a personal level, but I would ask that you refrain from making aspersions about the composers. That's unnecessary in this conversation.