One more thing that you should consider when choosing your GUI platform is the license of each GUI library and/or API .
Qt for example, has a dual license:
0. A paid-comercial license provided by digia for pure proprietary software.
1. A LGPL license that forces the folowing restrictions (for commercial applications):
0. Expose the interfaces for the free software components to end users.
1. Povide your app's object files.
2. Encapsulate the free software components inside a dynamic link library.
(If and only if you distribute your software to others)
GTK+ which is really brilliant, has a LGPL license as well.
WxWidgets has it's own license that lets you release your proprietary software without the need to expose anything as long as you disterbute your app in a binary form.
A word in regards to the win32 API:
The windows API gives you, by far, more power and flexibility than any other API listed (in the cost of implementing a huge set of things by yourself),
for Windows operating systems.
It is true that it has a quite steep learning curve, although it is brilliantly documented, rich, fast and beautiful !
Also, it is required if you attempt to create a pure DirectX application.
+ you can release your GUI components without any restrictions.
MFC is quite legacy and has an ugly interface (in my opinion).