In 4.5 RC 1 we hit several driver bugs again. We fixed a bug which caused blur not to work, but that fix intoduced different problems with some drivers.
It is possible that effects get suspended directy on startup. In that case try disabling the blur effect! If this does not solve the issue disable direct rendering in advanced effects tab.
There are also some problems with taskbar thumbnails and present windows. The previews might be upside down, too bright or the effect might be too slow. The same problem does not appear in desktop grid! This is caused by driver bugs already fixed upstream. So please update your drivers. You can disable the filter which is used for this in the global effects level, but this will also disable animations in Plasma.
We have bugreports for all those issues. If you want to comment on them please note driver and version.