MacOS is a hard creature to make apps portable to - as its UI is significantly different to most other OS's, that making a cross-platform library which appears to work as a native OSX app is hard, real hard.
I don't know where you got the idea that the Mac OS X UI is significantly different from most other OSes. It is more alike than it is different, and I think you are misinformed (or, more likely, that your information is out of date). If you can provide some specific examples, I will be happy to try to correct you; However, I don't think that discussion belongs here.
[ The discussion certainly belongs here - this discussion is about cross-platform GUI toolkits, which is a subset of cross-platform toolkits in general, which is very much a Boost topic. ]
That said, I think that your argument here is really that if you start by designing a framework without understanding how Mac OS X differs from other OSes, your framework will have trouble porting to Mac OS X. This is hardly a surprise. You can argue that way for any platform -- it's hard to port to a platform you don't understand.
Unless I missed completely missed something about OSX, you would preferably code it in Objective-C (which is a fantastic language for creating GUI's). In which case, GUI's for most other platforms are coded in C, C++, Delphi, VB, etc. wouldn't have a place on OSX. Is the prefered language to program GUI's on OSX, C++?
Take Mozilla/Firefox/OpenOffice - each of these suffers the "not quite OS integrated, compared with native apps", even though these apps are propably some of the highest profile cross-platform applications.
The quality of the user interface of all of those applications is something that I, as a professional Mac developer, would be ashamed to ship. (They are not all equally bad, though.)
you highlighted my point exactly - they are bad, compared to native applications, because it is _really hard_ to make a cross-platform GUI library. They are good, particularily considering this fact.
I firmly believe that the problem of producing a cross-platform framework that is able to accommodate the user experience demands of several platforms is difficult and tractable.
tractable => "Easily managed or controlled; governable" (dictionary.com) is the problem "difficult" or "easy"? (my guess that you have miss-type the last bit of the sentence - although I'm not sure which way it was meant...) Mathew