While I love Linux and I use it everyday, mostly as a server, everytime I enter in the desktop interface I do not feel the "home sweet home" I do on a OS X or Windows.
Linux DM are not comfortable, specially when you have KDE apps mixed with Gnome Apps and there's that mix of different gui + usability styles.
I've said this once, and I will say it again (running the risk of serious bashing): Linux should concentrate in only one DM and improve it.