I was wondering if there is something like WPF(Windows Presentation Foundation) for QT or KDE? And if there isn't, some other things I can consider to make nice GUI's.

I'm asking because, I like WPF, it is flexible and powerful. But it's windows only...

You mean like a GUI Designer + code that helps you to build animations in a beauty way?
there's QT Designer for gui building, and you may want to look at Plasma, since it already is able to do very good things, and there's an abstraction between what you see from what it does ( plasmaengines and plasmarunners if I remember correctly )