- Coherency: the way it works and is presented does not change much over the years. It's important for me. I don't want to adapt everytime a designer thinks he has a good idea.
- Simplicity: it just works and is well configured out of the box
- Efficiency: not the lightest Desktop Manager in the market but it is light enough so it won't eat resources for nothing
- Right level of complexity: there is enough to configure it to adapt it to your needs but not too much to make it a mess. Take the xfce console for example: you can rename tabs, move them and search text in them. Last time I check these features were not available in Gnome Terminal. But at the same time I don't want much more than that.
Coherency is really important for me: Xfce has been there when Gnome / Unity / Gnome 3 / Mate / Cinnamon were confusing me by offering all the same but not really the same.
Edit: I wrote "consistency" instead of "coherency" but I guess the right term is coherency in this context.