KDE? Aghhhh...
Lets use a free (as in freedom) kernel and then use a proprietary DE on top. Wow, we're all so intelligent throwing our freedoms away like that. [/sarcasm]
For those of you who don't know what I'm referring to allow me to explain. KDE uses a toolkit called Qt. The latest version of which is released under two licenses: The GPL and a proprietary license. For all GPL'd software Qt can be used freely however, for proprietary programs, Trolltech charges a hefty licensing fee.
The toolkit used by GNOME and XFCE is GTK+. This is released under a single license: The Lesser GPL. The lesser GPL allows proprietary programs to link to the code. You might be thinking, what's the big deal? Proprietary programs shouldn't exist anyway. Well, think of it this way. Each one of you who uses KDE is sacrificing your rights to your Desktop Environment and filling Qt's pocket, whilst endorsing proprietary licenses. Do you really expect companies to distribute freeware (proprietary but free software) that uses Qt if it's costing them to do so? Of course not.
Now lets move onto what the actual desktop environments provide us with. GNOME is simple, clean and ultimately customisable. Not to mention it's compatible with Compiz Fusion. The fact that KDE has issues with this makes me laugh. Who would want to use a Desktop Environment that doesn't support the graphical effects that Linux systems boast?
GTK+ is the standard. Both GNOME and XFCE use it. All major programs and multiplatform applications use it... >.>... use GNOME or XFCE over KDE, already.