The user interface in Windows Vista simply doesn't feel finished. The technology seems like it is there, but the user interface lacks consistency and the final finishing touches, an subsequently feels a bit pre-release - not the final release.
Windows Explorer doesn't have toolbars any more, no problem I suppose until you want to go up one folder. This is beyond me, you have backwards and forwards (but it isn't a web-browser), and no toolbar of common functionality.
Most applications haven't even been touched from their earliest incarnations, see Notepad, Task Manager, Paint, and loads of others. These are bizarrely much easier to use, but also help to confuse you more due to the lack of overall consistency.
In most applications the menu bar is hidden until you hit tab then it magically appears. If Microsoft don't like it then they should have simply excised it from all of the default Vista applications. I imagine lots of applications with custom drawn menus look pretty weird now.
Tabbed property sheets and toolbars look really weird when you let Windows Vista perform the rendering. The colours aren't quite right and this makes it look unprofessional.
Firefox looks great in Windows Vista though - much better than it does under XP, and I was pretty happy with it there.