closed as not constructive by Oded♦, Yannis Rizos♦, Jonas, Glenn Nelson, ChrisF♦ Dec 3 '11 at 18:44
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Mac applications with a GUI are usually written in Objective-C and use AppKit, which provides a UI that's consistent and similar in every application (e.g. short cuts are the same everywhere, the same controls have the same size, dialogs are the same, menu bar is similar in all applications). This means that if you know how to use TextEdit, you automatically know how to use TextMate, and if you know how to use iTunes, you can find your way easily in iPhoto and Aperture.
Qt breaks this consistency by using controls that aren't native. For example, using windows instead of sheets to show an error message, or by not using proxy icons in title bars of windows, or using them wrongly. I do use one Qt app on my Mac, which is the GUI front-end of Doxygen. It works but the first time I opened it I immediately saw it wasn't a so called "Cocoa" app (one that uses AppKit as described above).
If you want to have a cross-platform app with a GUI consistent on both platforms, rewrite the GUI for each platform. You can use the same back-end.
tl;dr: If you are writing a little tool like a front-end for a command line tool, or a simple calculator, go on and use Qt. If you're writing a big application like an IDE, a Terminal emulator or a Twitter client you should rewrite the GUI completely for every platform to be consistent.
On windows it's native - themes and everything. Rather more native than a lot of the OS, which when you dig down into some of the management screens goes NT on you.
On Linux it is the native toolkit for KDE and looks ok on Gnome
Don't use Mac - but Mac users do get very touchy about everything looking exactly as Apple says it should