KDE is pretty feature-rich, but it is just ugly. Look at the poor use of spacing/margins, the ugly black & white icons, the ugly font, the square edges, and just the overall look & feel. It needs a major face-lift. I dislike the attempts to dumb down GNOME but KDE being ugly makes it really hard to switch to.
I'm gonna be very controversial here, but the problem is that the vast majority of Linux users are blind to aesthetics and design. Perhaps the vast majority of people are. Mind you, being blind to something does not mean it does not exist. Just look at the desktop wallpaper or any font choice of the average person. It's no wonder Comic Sans and Papyrus are the most common fonts in the world, even for formal business…
Yes, my god is KDE ugly, but just because they don't prioritize aesthetics doesn't mean they don't know better.
Linux users (and developers) prioritize functionality above all else. Meanwhile the entire discipline of visual design is just not very consistent. There are far too many subjective little rules and exceptions to maintain and they change all the time. Who cares?
Deliberate oversimplification of how things are displayed from the perspective of the implementer rather than the designer is its own aesthetic. It's the choice of reason and a rejection of the imperfections of human perception. It's rebellious and edgy. It's not a mere rationalization of laziness or ignorance, but a middle finger to business types and their marketing henchmen with their annoying design guidelines and requirements that just get in the way of the actual product. KDE was not made by people getting paid to smile and nod while staying quiet in their cubicles (at least not during the hours they worked on it). Very rock and roll. I'm all for it.