This blogger lists a bunch of reasons for why Ubuntu won't make it into mainstream – btw, he's an Ubuntu user.
Before I start, I would like to say that I am an Ubuntu user. I love it and I use it as my main OS. When I need to use Windows XP it is usually so I can use applications like Photoshop, Macromedia etc. (Yes, I know I can make them work under WINE, but they perform a lot better under XP). However, I am not blind to Ubuntus faults. It is a wonderfull OS, but this blog is not about that. It is about things that arent as wonderfull.