Ubuntu's biggest flaws, in my opinion, are...
1. Too much reliance on the Terminal (you can use OS X without it; completely the opposite w/U)
2. Lack of a truly attractive GUI (built-in, from the start, no Beryl/Compuwhatever.)
If they fixed these two, the OS would be much more appealing to Windows users (who, let's face it, are the people they're trying to convert). The first issue is a bigger deal than the second, but the second is still relevant. Basically, they need to go a lot further in simplifying the downing and installation of applications. When they get to the point where 99% of apps can be processed the way you do so on Windows or on OS X, people will truly be able to just download a disk, burn it, and install the OS. Until then, it'll remain something you need tech support for, which limits its marketability.