Besides, what do you think is so horrible about Vista's UI? I've extensively used both Gnome and KDE, and much prefer Vista's UI. Usability is one of Linux's biggest weak points, IMO.
I think the issue is that people can't think of anything better to implement.
I hate to be critical, but those desktop & taskbar icons just ruin it. They are like the retarded child of apples icons, and you can almost guarantee they were drawn by a programmer not a designer.
I use linux every day and wish a great designer or two would work with them and gnome to give it the "polish" of other operating systems. A pixel here and there makes a huge difference if you ask me.
Other nitpicks: taskbar text should be vertically centered. Consistency in font appearance is a must. Text on the mail window seems to be off vertically by a pixel or two. The clock text is far too prominent. Etc etc etc...
Maybe users like it?