I was playing a bit with KDE on my work computer running Ubuntu 10.10 and was wondering why it was a bit sluggish and didn't look that good even though the computer isn't that slow. Turns out desktop effects were disabled and I couldn't switch them on and there was no actual error message stating the reason.
Solution was simple:
- (backup and) delete ~/.kde/share/config/kwinrc
- kwin --replace
Running kwin on console printed out info about it finding NVidia opengl drivers and enabling desktop effects.