I've been using Linux since '94 and I've used nearly every major desktop environment at some point along the way. In my experience, the biggest problem KDE has isn't features. KDE is everything I actually want in a desktop (other than using minimal resources like traditional window manager fluxbox). The problem is that the environment is buggy and/or unstable. Every once in a while, I will try to use KDE as my desktop and I will only last a few months before going back to Mate/gnome2, XFCE or something like icewm. The list of odd behaviors would be too long to post here but I've had endless problems with Kopete, Kmail, Korg, Konq (browser) and many others. In comparison, I can leave up a gtk based desktop for weeks to months at a time without thinking about it with a similar compliment of apps (Pidgin, Tbird, FF, etc.) and rarely run into any weirdness.
They have the features, they just need to really nail down the stability, clean up the cartoon fonts, and set the default settings to something more usable. Admittedly, I'm overdue for a test run of plasma 5.