I like that KDE applications are at least somewhat consistent in their UX and in the way they organize basics. For example, the way settings and shortcuts are handled is identical across all apps. Kparts makes embedded viewers consistent and good across apps. Support for GTK theming reduces the visual annoyance of the rare GTK app. KWin works very well, has a broad selection of useful shortcuts and has been the most stable compositor I've ever used (AMD/open source).
On the other hand, KDEPIM/Akonadi seems like it is quite powerful, but for some reason I kinda didn't became comfortable with it. Not sure why, exactly. I used KMail and KOrganizer for quite a while, though. Decent tools, but some rough edges when it comes to first-time setup, e.g. the wizards could really use some polishing.
There is a surprising number of useful and powerful utilities in KDE.