it was mainly audio apps that I was talking about. A lot of them already use Qt.
if you look at as project like lmms, i don't know how you could do something like that without qt. i'm thinking of the bars that represent sound effects... i suppose in gtk (where i'm not certain, but i think you'd be restricted to one color for "3d effects, and it's a color that gtk_prefs would control instead of the application's author...") you could have 3d buttons for the "bars" and give each one a solid color "face" (think of css, which doesn't let you change a 3d button itself, but it lets you change the color of each side) to distinguish it from other bars.
in other words you can have purple 3d bars and red 3d bars and blue 3d bars in qt, or gray bars with a red 3d rectangle, gray with a purple rectangle, gray with a blue rectangle, but you can't actually have different colored 3d bars in gtk in the same app.
not that you have to use 3d bars. you could just use flat rectangles for each effect. but what i'd really like to see is a "meta kit" for gui projects that wraps gtk, qt, and gnome. if you built an app for this kit, it wouldn't translate every qt feature to gtk, but if you used the features of meta kit, you would only use the features of meta kit.
the features would range from gtk features (and gtk compromises of qt) to qt features, but no matter what you built, it could be rebuilt automatically as using only gtk or as using only qt (or only gnome...) just by changing the compile options. then a project like 'k3b" (or perhaps, m3b) would be a project that you could compile for gtk or qt or gnome, it would be a sort of universal wrapper tool kit- for the coders that wanted their apps to be that flexible. you could even optimize by having several wrappers, and which one you used depended on which toolkit you're compiling for.