KDE 3 was released only a few months ago, and it is, to date, the most successful version of the series, serving more than 50% of the Unix and Linux desktops, surpassing Gnome (~21%) and the rest of the gang. However, KDE is not perfect, and still not as comfortable as the Windows or as sexy as the MacOSX desktops. It lacks two things: integration with the underlying system and UI polishing. Today, I will mostly talk about the polishing part, as a lot has been already said elsewhere about the seemingly unsolvable integration issue (because of the modularity and completely independant/remote software projects.)
Update: And as I was just publishing this article,
KDE 3.1-Alpha was released. I hope that some of my recommendations will make it to the final version of KDE 3.1.