In your examples, you are comparing KDE apps with GNOME apps. Though the KDE ones are all Qt and the GNOME ones are all GTK, it's not really a fair comparison since GNOME and KDE have different design objectives (KDE strives to be more feature-complete and configurable while GNOME wants to be simple to use and have a clean UI). The design choices in your comparison aren't dictated by the toolkit, but rather the HIG of each desktop. There are a lot of clean and simple Qt apps, like the official implementation of Tox [0].