
Java Swing comes with "pluggable look-and-feel technology", which essentially boils down to the fact that interfaces can be "skinned" (although this is simplifying a tad) and is therefore, extremely flexible. By default, Java ships with a cross-platform look-and-feel (LAF), which means your apps can look consistent across all platforms, or LAFs that mimic the look of a specific platform, say Windows, for example. However, one of the chief complaints of Java desktop applications is its "look". It basically stems from two issues:
Maybe they should just include a Mozilla module and have everyone define their UIs in XUL. I think I heard that something similar was coming to Mono - that would be even better, to define the UI of an app in the same way in both Java and C#.