I guess all those years I've spent with Solaris and Mac OS X have been wasted
I never said GIMP uses X11 on Windows - I said it's a port of a Unix/X11 application. I should have said Linux. Excuse me.
This is semantics and it doesn't matter, my point was Gimp uses Windows native buttons and controls when you run it under windows (and it's not themeing, it uses the same GDI+ API as all other Windows applications).
Bottom line is most Windows and Mac users are accustomed to MDI style applications with one menu bar at the top of the screen that controls the foreground window, and this is not the way GIMP behaves.
I would disagree for the Mac part, having spent quite a lot of time using a Mac. Many applications, including Apple iWork suite in fact use a single menu bar, floating palettes and multiple windows for all documents open, much more like the Gimp than like Photoshop.
Besides, if MDI were the actual issue, people would just use Gimpshop. I think it is usually wrong to try to replace something that works well enough for you (including Photoshop). If you've paid the license already, and you enjoy the software, even if Gimp had more features, there's no reason why you should change.
Now if you want to use Gimp, it'll take you some time to get accustomed to the menus, the shortcuts, and eventually to create your own config, but if you believe it's worth the time investment because you don't wan to spend money on photoshop and you'd rather buy lenses instead why not ?
