There is a lot about GTK I like; it's a C library making compatibility easier[1], it always seemed way faster (less memory bloat?) than QT, and while the widget-packing/nesting style was somewhat ugly, I found it surprisingly easy to write.
I was even enjoying Vala. While it was obviously a young language, it had a lot of interesting ideas.
Then we got the new 3.x version of GTK with its "lets rewrite everything for no other reason than to break compatibility"[2] project goal that seems to have corrupted far too many projects recently. In addition to the problems already mentioned in this thread, it seemed so.. unfinished. Various components or features were gone or rewritten into something else. I guess they spent all their development time trying to tie Gnome and in as tightly as possible instead of finishing features.
The last straw was when they decided to join Pottering's "lets forget Unix and make Linux into Windows" crusade. A terrible design decision on top of years of other questionable choices and bad attitude about actually listening to user needs.
I supported GTK and Gnome waaaaay back when they first started, when the fight was between a Free (GPL) library and the increasingly popular proprietary-license-only[3] Qt. Now, I'm not sure what to support in the GUI toolkit area.
While I figure that out, my current project's GUI is being written in ruby-tk. The widgets in Tk have a terrible look and strange layout/interaction quirks, but at least it isn't a moving target and work more or less everywhere.
[1] e.g.: writing Ruby bindings C++ libraries can be problematic. While problems such as the name-mangled symbols are not as bad as they once were, it is still much easier to link a C library into a random environment.
[2] As Linus said, "...thou shalt not break working code."
[3] Trolltech changed the license about a year (?) afterwords.