For those of you who don't know, QtCreator's "widget promotion" feature is a handy way to use custom widgets in QtCreator without having to compile a plugin. The problem on Windows is that QtCreator is compiled with MSVC, but it ships with the mingw compiler. Well, QtCreator's plugins must be compiled by the same compiler as QtCreator. In short, if you want to compile a QtCreator plugin, you must either recompile QtCreator from source, or just use widget promotion. I've done both, and let me tell you, widget promotion is easier, quicker and simpler.
If you'd like to see an exhilarating tutorial video, I've uploaded one to YouTube.