UI is important, one of they factors to the success of a application. Users always seem to like applications which are "user friendly". This drives developers to work more on the UI's and also to follow some common standards.
<- [Screenshot GNOME on Linux]
Windows seems to be the most leading OS when it comes to UI's. I will agree to the fact that OS X is far better. The reason why I would say Windows is leading, cos its the most acceptable thus far to most people. The concept of software should be seen more as cars. Once you learn how to drive one car, you should be able to drive the rest. Does not matter who manufactures the car. If this was the case, it would be perfect, slowly I think this is becoming a reality. Web applications have taken a good side to this. Most web based applications look and feel the same, regardless of the OS. This is also true for Java based applications. If this was the case for all the rest; things would have been so much better.
In the Linux world, the UI issue is not really going where it should go. I agree over the last 3 years or so we have seen a major leap in the GUI's. KDE and GNOME has improves a lot. But the fact that KDE is based on QT and GNOME is based on GTK again raises so many issues, which should not be. Application portability has been limited due to the presentation layer of applications. Most of the code relating to the business logic and others can easily be ported, but when it comes to the UI's, there you get the problem. If only all OS's followed some basic standards. Life would have been so much easy. Maybe I am dreaming.
[Screenshot of KDE on Linux]