Gnash and Epiphany
Long story short, the proprietary Adobe Flash was blacklisted in the GNOME 3.x releases of Epiphany because it uses Gtk+ 2.x (or 1.x) while Epiphany uses Gtk+ 3.x. An unwanted side effect of this is that it also disables Gnash, depriving me of my daily dosage of Youtube, Rebecca Black and crappy, obscure Bollywood songs.
So I fixed it. It is probably not the most correct way to do it, but then it can be a bit tricky to define what is correct in cases like this. In any case, GNOME Bugzilla #647516 has the patch and some details. (Thanks to Company for all the nitty gritty details about this.)