Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Having switched from Ubuntu to Kubuntu recently, the first thing that irritated me beyond all reason was that single-clicking on a file or folder immediately opens the file instead of selecting it. Since I use Windows and Ubuntu on a daily basis, it's just frustrating that it works differently in KDE.
The one thing you can't complain about, however, is how nice and easy KDE's system settings are. Just open them up via the "start" menu, and find the Keyboard & Mouse icon.