On my 64bit systems Alt-Tab and other key combinations are not working. ‘kwin --replace &’ solves it for the current session.
See: https://bugs.kde.org/show_bug.cgi?id=217657
Now installing the packages from Factory, see if it’s the same with 4.3.80, met some odd dep. problems, already described elsewhere on the forum.
I also change to Factory from Unstable and weird thing happened, the “Plain Desktop” option disappeared Also Alt+Tab works now without kwin --replace;)