While Qt not being C++ cannot be denied, I have to correct you on the
reason. Trolltech deliberately chose not to use many C++ features in favor
of better portability and I have to agree: that did make sense in the late
90's when C++ was just being standardized and when the compilers were
nowhere near supporting it properly.
Today it is a different situation. With some magic of templates one can
implement really nice signal handling that is safer and a lot less
intrusive than Qt's MOC/QObject-based system. SigC++ of GLibmm and
Boost.Signals are the best known implementations and both have very similar
features. Both can use each-other's "signals" just fine because any regular
function or functor of the right form works.
From a developer point of view, GTKmm is by far the nicest GUI library to
work with. It is elegant, it gets the job done with no extra boilerplate
and it doesn't leak memory or segfault (nor use pointers, what a
coincidence) in your everyday GUI stuff. It's a shame really that it
doesn't offer native UI on Windows or Mac and thus using it is really not
an option for cross-platform software.