Thanx Husse, but that was not it.
When you change brightness or volume, Compiz (I think) pops up this little applet that shows you a scrollbar how high the brightness or volume or whatever you're changing is. This used to work right out of the box, so I don't know what package does this. Which package is that?
I don't even know how I got into this mess, and a complete reinstall seems like overkill.
Don't fix it ...., don't break it ...