IIRC, it used to be that even the core Qt libs were GPL (unless you paid for a commercial license), while now most (but still not all) libs are also available under the LGPL.
GTK has always been plain LGPL 2. Nothing scary.
KHTML/WebKit: I don't think the GTK/Gnome folks ever wrote a browser engine. KDevelop: Did the GTK/Gnome folks ever write a significant IDE with a code model? Krita: It's focused on painting/illustration and doesn't directly complete with Gimp. Among artists doing this Krita is likely more popular than Gimp now, since the latter isn't even in the running. (Support for MyPaint brushes is progress on that front, though painters want many other features.)
For KMail/Kontact there's Evolution, but arguably neither ever became really popular if you compare them to, say, Thunderbird.