Just to illustrate the USE=gtk3 confusion, on packages I has personal
experience with:

app-i18n/uim
x11-themes/light-themes
--> flag provides support for gtk3 apps, in addition to gtk(2) (with
independent USE=gtk in uim); most users would probably want this.

gnome-base/librsvg
--> flag for gtk3 libraries *and* executables (independent USE=gtk; an
example of a package that should be slotted?)

media-sound/audacious
--> REQUIRED_USE="^^ ( gtk gtk3 )" with default switching from version
to version (current stable is gtk, previous was gtk3).

www-client/midori
--> USE=deprecated instead of USE=gtk3 in unstable.

www-client/uget
--> for once, a "simple" USE=gtk3 that enables gtk3 instead of gtk
(gtk3 support still experimental?)

media-libs/libcanberra
--> USE=gtk3 enables extra support, in addition to gtk: "Enables
building of gtk+3 helper library, gtk+3 runtime sound effects and the
canberra-gtk-play utility. To enable the gtk+3 sound effects add
canberra-gtk-module to the colon separated list of modules in the
GTK_MODULES environment variable." — very unclear: is it needed?
recommended? also, why doesn't the package handle the environment
variable by itself?

-- 
Maxim Kammerer
Liberté Linux: http://dee.su/liberte