Some programs eg: certain versions of toonboom, will not run at all without QT installed; some programs may run but certain functions will be lost; in the main it's programs that have been ported over to windows requiring QT for internal video functions and such

do what mike says regarding plugins and settings - then make an informed decision - pedestrians get killed on roads; so if you don't need to cross the busy road don't, but if you do, cross it with your eyes open