Comment 5 for bug 619578

For info see also:
http://blog.martin-graesslin.com/blog/2010/09/driver-dilemma-in-kde-workspaces-4-5/
http://blog.martin-graesslin.com/blog/2010/09/demystifying-opengl-desktop-effects/

So to summarise according to Mr. Graesslin some drivers incorrectly report support for features that they do not support properly at all, and -as I understand it- when kwin tests the reported functionality it crashes. This crash is then detected and kwin disables desktop effects.