I'm eager to get transparency support for top-level AWT/Swing containers.
The three limitations that have always frustrated me about AWT/Swing are:
1. No task-tray integration (now available via third-party extensions, at least).
2. No "always on top" windows (fixed in Tiger).
3. No transparent windows (still waiting!).
People like to complain that Swing makes it too complicated to do some things, or that it's too slow at others, but these are the three things that, until recently, were simply impossible in Swing.
Anybody care to add to the list?