I\'ve noticed that GTK has a uniform feel across all platforms, which is great for Linux lovers, but not so great for people who prefer their native GUI feel.
I have
Out of Qt and WxWindows from a clean slate I'd go for WxWindows - simply because it uses the native controls which gives you better support on future platforms.
I arrived at this decision by the following unscientific process:
For me, the above is easily enough to go with WxWindows as a first choice and see where I end up after a while.
Worth mentioning that I've been doing a lot of XAML and so I'd definitely be looking into using XUL instead; as it is much nicer to get the layout out of the code.
As a footnote years ago I developed an application using the thankfully long-dead and dreadful ZincApplicationFramework - which layers it's own controls ontop much like Qt. It was dreadful and I spent ages messing about with the internals to get it to work right under Windows. This makes me prejudiced, or a least skeptical, of anything taking the same approach, because shortly after this project finished I used Allegris C++/Views to create another application, and this was much better because it was a very thin layer above the native controls - which meant that it was easy to do things and a lot more efficient.
So I may have a preference towards thin layers and native controls....