Complaining about the appearance of a modern OS is an invalid complaint, because you can always tweak an OS to look however you want to some degree. If you don't like the default theme of KDE (or any DE for that matter), just change it.
This is the same problem with Ubuntu. Some people really hated the orange theme and didn't want to use Ubuntu specifically because of it, which is utter crap because first of all, the orange made ubuntu distinctive and secondly, just change the color if you don't like it.
I suppose they could call it that, its boring but it works.
Actually, I used XFCE on a very crappy system and thats how I knew it didn't perform much better than GNOME 2. But again, I haven't used it recently so I'm not sure how much it improved. IMO, LXDE is so much better at doing what XFCE tries to do. Its simpler, cleaner, a lot less CPU and RAM consuming, and more sleek. LXDE does lack many features, but it gets the job done. According to one of the LXDE devs, they said what really separates their project from XFCE is how LXDE tries to operate with as little dependencies as possible. XFCE, from what I've noticed, is littered with GNOME dependencies.