A couple of notes on Qt.
Qt has for a while uses GTK on gnome and other GTK based desktops as the native widget set.
Qt does do Mac native but not using the same code path as you would use on other platforms, You are going to need to a reasonable amount of version(mac)...else
Practically every commercial application I have purchased that runs on Linux in the last 5 years is Qt Based (or rolls their own UI from scratch), usually they control the theme, colour scheme rather than the system one.

IMO Qt is currently the least bad option when it comes to making cross platform productivity applications. Having stable bindings would be a huge help for D.