I'm writing this just in case someone else runs into the same issue.
Today when I booted my (Linux Mint 15 KDE 4.11) desktop the window decorations didn't show up, I wasn't able to move windows or change their focus and the keyboard was unresponsive in Native KDE Apps (i.e. the keyboard worked in GTK apps like firefox and synaptic). I messed around trying to figure out what the problem was - previously when I had problems I would restart plasma and things went back to normal, but since the windows decorations were not displaying I suspected the window manager. I was just about to install kubuntu instead then I thought I'd have one last try at looking at some packages - good thing the keyboard worked in synaptic otherwise I wouldn't have bothered. I noticed that kde-window-manager was not installed. I clicked to install and and it said it need to uninstall kwin-style-crystal. I remembered that a few days ago I installed a few themes and icon sets to try them out. Reinstalling kde-window-manager solved the issue. I don't understand why but the kwin-style-crystal and kde-window-manager packages are mutually exclusive; installing the kwin-style-crystal package might make your system unusable. Just be careful not to remove kde-window-manager.