I haven't played WoW, but I have 2 IRL friends who quit XI for WoW, so I only comment based on what I've heard from them and I might be wrong in something. The basic thing is, the game seems too easy. You can do everything yourself, level crafts to insane levels effortlessly, and just level off of quests the whole time. It just doesn't strike me as something that I'd really consider worthwhile. One of my friends was over 40 in about 2 weeks, that's the kind of thing you can only do in XI with PL's and statics, far too easy.
I'm proud of some of my accomplishments in XI, even though thousands of people have done them before me, because it took time to do and a lot of work. If I can just log on and do it within an hour all by myself, it doesn't feel the same. I do like the comments that have been made about how WoW seems to be better suited for casual players, which is probably true.
But one of the biggest draws of XI is the community that's been created from everyone partying with each other. If I need help on something, I've got friends or acquantances that I can /tell for help. Sometimes it's a pain to get enough people for some of the larger/harder/obscurer events, but you feel like you've accomplished more for the amount of effort it took to do it. My friends who play WoW said that for the occasional stuff you need a group for, it's a lot harder to get people. It seems like the community in WoW, like the rest of the MMO's I've heard about, isn't nearly as developed or as strong as the one in XI, simply because we're "forced" to party and get to know each other.
There are some nice things about WoW they've said, like people can't monopolize some NM's, when you go into certain areas, you get your own version of the area, with your own mobs to kill. But then, it just means that everyone will have leaping boots, emp pins and ochudo's kotes, which detracts from the pride of having them.
I also don't like the graphics, but then I fucking HATED warcraft 3. The single player was just boring as hell and I just started cheating to beat the levels and get to the story, just so I wouldn't have to suffer through another mission.
Regardless, WoW has some perks and might be better for some people, but I'm not leaving XI for it. I'll take my hard-won prizes anyday.