Probably a coincidence, but after applying the fix to How to get the proper LightDM theme? and running the latest batch of updates, my desktop suddenly looked like this after rebooting:

Funny thing is, it seems to be account specific, as my girlfriend's account seems unaffected. I can't logout because the whole entry is missing on the top right. Rebooting doesn't solve the issue. System settings (when run from the launcher) lacks a whole lot of icons and changeing the theme doesn't solve the problem (only changes the window top bar).

I'm going to try the solution in How do I fix my theme? first, but if that doesn't work (likely, since I seem to have a slightly bigger problem than the one in that question) I'd love something else to try (other than creating a new account).