I have just upgraded to Linux Mint 17.1 Cinnamon 64bit.
For various reasons, I did a complete format and install, so the following may not affect those just doing an upgrade.
There is a small problem with running some KDE applications. As an example, I installed KNetWalk [a small timewaster!] and this is the menu bar that is displayed –
With a bit of research, I found the answer as submitted by Clem.
sudo apt install kdelibs-bin kdelibs5-data kdelibs5-plugins
To install the icons –
sudo apt-get install oxygen-icon-theme*
Rerunning the application, this is the result –
Just in case anyone has similar problems…….