Every DE except LXDE and Unity has major priority issues (not that Unity doesn't have issues, but it is moving toward its goal):
Both GNOME and KDE keep focusing on things that they never had to touch in the first place, such as how they both have their own office suites, web browsers, instant messengers, music players, etc. - things that have no reason to be DE centric. This particularly annoys me because none of these types of programs are better than the DE-neutral alternatives, and many distros include them due to being the default or more integration. I don't have a problem with a core program having different GUIs depending on the DE, such as recordmydesktop. There's a GTK and a Qt version of that, and I think that's perfectly reasonable.
As for other DEs, Enlightment focuses more on special effects than functionality and XFCE keeps tacking on features while somehow self-praising itself as being light-weight. I personally find XFCE's default programs (such as text editors, file browsers, etc) to be the worst of all DEs.
I'm currently only using LXDE right now because its the only DE I have found where the developers strictly focus on improving the GUI and nothing else. I don't want developers spending time on half-assed programs and features with blatant superior alternatives. It just divides the attention to things that really matter, the things that everybody of a DE would use.