The thing I think is going to hurt Kinect's legs the most still the fact your required to stand up and actually interact with all the games. Most people when they play videogames like to sit down and relax, not jump up and go crazy. This is why all the motion controls on Nintendo games for the Wii only requires you to do very simple movements to do them so that anyone can play the system. Now yeah, there are some people who might get up and swing the controller around, but for everyone else, a simple flick of the wrist can do the same thing.
Now I do expect Kinect to sell well for these early months based on all the hype Microsoft generated, but after the holidays season is done I expect a lot of people to start getting sick of the thing rather quickly and the word of mouth to get poor. I knew people who when Wii Sports came out, would stand up and play it all the time for the first few weeks because of the early excitement. After a while though, even though they still liked the game, they started playing it sitting down because the original excitement of pretending to be a real Baseball or Tennis player wore off and they just wanted to simply play the game for the gameplay and didn't care about the novelty of pretending your a real person anymore. This is why when Wii Sports Resort came out, they still bought it because they liked the first Wii Sports but played all the games sitting down when they first played it, even though a few years earlier they were jumping all over the place when they first played the original.
This is why I see this being a major problem for Kinect. With the Wii, once people got tired of playing while standing up and waving their arms around, they had the option to sit down and still enjoy the games in a more relaxing and simple setting. With Kinect on the other hand, once people get tired of moving around, they'll have no such option and so will be unable to play and just stop playing the games altogether. As a result owners of Kinect will stop buying software because they don't care about standing to play anymore and they'll tell their friends that the system gets old fast which will kill the momentum.