I do not have an answer but I would like to show a different angle to this question: why do Qt applications often feel overly complex and GTK applications are usually much simpler to use?
Concrete examples:
* Transmission ( https://www.transmissionbt.com/images/screenshots/GTK-Large.... ) vs. KTorrent ( https://www.kde.org/images/screenshots/ktorrent.png )
* Rhythmbox ( https://wiki.gnome.org/Apps/Rhythmbox/Screenshots?action=Att... ) vs. Amarok ( https://amarok.kde.org/sites/amarok.kde.org/files/Amarok2.7s... )
* Simple Scan ( http://i1-linux.softpedia-static.com/screenshots/Simple-Scan... ) vs. Skanlite ( https://docs.kde.org/development/en/extragear-graphics/skanl... )
* GNOME Maps ( http://www.gnome.org/wp-content/uploads/2010/09/maps-3.12-11... ) vs. Marble ( https://marble.kde.org/img/gallery/marble-desktop-atlas-dist... )
* Brasero ( http://www.novell.com/documentation/opensuse103/opensuse103_... ) vs. K3b ( http://digitizor.com/wp-content/uploads/2010/02/k3b-1-70-0.p... )
* Shotwell ( http://ubuntuportal.com/wp-content/uploads/2014/10/ubuntu-14... ) vs. Digikam ( http://www.flickr.com/photos/digikam/2603789332/ )
Please also compare the default theme for Gnome ( http://blogs.gnome.org/mclasen/files/2014/06/adwaita.png ) with that of KDE ( http://upload.wikimedia.org/wikipedia/commons/6/6d/Plasma_De... ).
I appreciate the configurability and power of most KDE applications, but I cannot stand the lack of design and ergonomics that pervades the KDE world.