I had this though for a long time. Although I believe Qt is superior to Gtk, there seem to be more innovative projects done using Gtk: there was no equilvalent to gimp, GnomeMeeting, AbiWord, Gnu Cash, Gnu Spread or sodipodi before they were started. And all in Gtk or Gnome. There is also the CD burning app (don't remember the name).
Apart from a few very ambitious applications, like KOffice and Konqueror, KDE seem to have tackled common applications. KMail and Konqueror for example are very good but we already had a mail client and a browser before. All vector drawing program (Karbon14, Krita) seem to have a hard time becoming usable.
However, there seem to be more and more applications that are better than the original one they were copying. Maybe it is just a subjective impression but it is mine.