It's the immersion in the world and community that keeps MMORPGers in our games, that makes us feel a connection to it. WoW is, to me, lacks the most in exactly those aspects.
Lots of loving care is going into raid content that few people ever see. Rep grinds and loot grinds abound. But where's that sense of fantasy and connection to the story and world, the sense that you are an inhabitant of Azeroth? To me, that is the most important aspect of an MMORPG. But currently, WoW is too focused on loot grinds and scripted static content that is consumed faster than it can be created, or is not consumed by most people at all. The next MMORPG that's looking to take a bite out of Blizzard's share of the market would do well to study these weaknesses and be strong in the ways WoW is not. Because there are many players (me included) that would consider leaving WoW for a game that could provide those things better, even if they weren't as 'polished'.
EDIT: I just found this post on Girls Don't Game, thats echoes a lot of my feelings:
"Not to knock WoW or anything, since I still play it and don’t feel like being a hypocrite today, but I really can’t get over how they seriously have not planned on giving us player housing. This is a horse that has been so incredibly beaten, you can barely even recognize the corpse. But it’s true. While players whine on the forums every single day about the lack of player housing, Blizzard keeps continuing to raise the level cap and add more high level instance raids left and right. There is so much more that could be done, in my opinion.
...I would love a game where I could just log on and not worry about hitting 20 that night, or farming primals. I would just like to log in, maybe redecorate my e-house, kill some monsters for new items, hang out with friends and then log out at the end of the night.
I heard that Blizzard was thinking about revamping the fishing skill or something but I have yet to see some actual hard evidence on this. Fishing is actually one of my favorite things to do in World of Warcraft. When I was still playing with friends, I wound up hitting fishing 300 because I was sitting on the dock in Orgrimmar all the time in private messages with them."
The open-endedness of MMORPGs is their strength -- it's what makes players feel like they're part of a world, and have many ways they can have fun in it. To run around in a fantasy world, with the feeling that you can find all sorts of different ways to make your place in it, is the main draw of these games. That is where WoW is lacking the most, in my opinion.