Sekel; to be honest I'm new to Mac OS X, and wouldn't know what it would entale to get it to run natively on Mac OS X.
I read about Carbon and QT4 - on those 2 I'm not even sure what exactly the difference is.
So specifying the exact components I'd need for my applications (miscellaneous freeware) would be quite hard.
You would need the same components regardless of which widgetset you use.
Qt looks great and is a true native widgetset, but has two requirements: Users need Qt libraries or framework installed (or these can be included in your app's bundle) and the opensource Qt libraries can only be used for opensource apps, otherwise you need a commercial license from Trolltech. Your freeware would probably qualify for the opensource Qt libraries if you make the source available too.
Carbon widgetset only requires the Carbon.framework, which is part of OS X. However, this widgetset is only in the birthing stage (i.e., not good for anything yet, but has potential).
GTK and GTK2 widgetsets are not native because they require X11. GTK looks ugly too. GTK2 looks much nicer, but it seems rather slow compared to GTK and of course the GTK2 widgetset is not finished yet. Both require rather large libraries to be installed on your users' systems (plus X11) in order to run your apps.
To see what library dependencies your app has, use otool with -L switch on the executable file.