The closer to the metal (so to speak) that you are programming, the more difficult things get. WinForms (provided by the .NET Framework) is a pretty outstanding abstraction over the Win32 API, considering the complexity you've already seen that it involves for the even the simplest of tasks, like getting a window to appear on the screen. All of that is still happening in the background, of course (registering a window class, creating the window, etc.), you just don't have to write the code yourself.
It's interesting that you write off Mono as a "hack", but would consider a library like Qt. I'm really not sure on what basis you make the distinction. The Mono library is widely regarded as excellent when it comes to WinForms support. The biggest detractors are the same as Microsoft's own CLR implementation, namely that it doesn't produce truly native code, which is more irrelevant to performance in the majority of situations than one might think. Beyond that, some complain that Mono applications don't conform fully to the platform's UI guidelines (that is, they don't look and behave exactly like a native application would), but I have a similar complaint about applications written using Qt.
It seems like literally everyone recommends using Qt if you want to do GUI work in C++. As I mentioned above, it happens not to be my favorite library because I'm a stickler for using fully native controls and widgets provided by the platform you're currently running on. I understand that Qt has gotten a little better at this recently, but I still don't think it's up to my standards. If you're more flexible than I am (and I'll warn you that the average Mac user is not any more flexible than I am), and true platform independence is a big concern to you, it's probably the one you should opt for. Many people praise it for its design elegance and convenience, although I seriously doubt that even it offers the same simplicity as the .NET Framework's implementation.
If sheer simplicity and terseness of code is as important as the beginning of your question makes it sound, I highly recommend sticking with C# and WinForms. Things get harder as you start to remove layers of abstraction, and if you don't need the extra levels of control that doing so affords you, there's hardly any justification for making more work for yourself. Mono's Forms implementation is a perfectly viable solution for cross-platform applications, assuming your needs are relatively modest.
Beyond that, if you want to create a truly cross-platform application in C++ the right way, I recommend that you strictly separate your data layer code from your UI layer, and then write the UI using the tools provided by each platform you want to support. In Windows, your options are relatively open: .NET WinForms is a solid choice, native Win32 is a somewhat painful though merited option, and a handful of other libraries like MFC and WxWidgets can help to ease the pain of fully native programming (though not nearly as well as WinForms does). On the Mac, the only real option is Xcode, Interface Builder, and Objective-C, targeting the Cocoa framework. Linux/Unix-based systems are hardly my forte, but I'm given to understand that Qt is about as native a library as you can get. This sounds like more work than I think it is—a well-designed library should handle 80% of the work, leaving only around 20% that you have to do in implementing the UI. Beyond using truly native controls and widgets, I think the other big advantage afforded by this approach is flexibility. Notice how Microsoft Word looks very different (despite some superficial similarities) on Windows than it does on the Mac. And iTunes has become almost a paragon of excellent UI design on the Mac platform, but sticks out like a sore thumb on Windows. On the other hand, if you rolled out something like Windows Media Player on the Mac (and yes, it's been tried by Microsoft themselves, though without much success), Mac users will dismiss it as a complete abomination and probably be somewhat offended that you even tried. Not so good for the truly cross-platform-minded developer. All of that to say, if your app is anything but the simplest of utilities, you'll probably find that an entirely different interface is justified (and even expected) on each platform that you want to support.
No matter how great Qt may be, you're not going to get that with it.
