All great, except a large portion of apps are still Gtk2 based, which means any potential theme developer needs to make his theme at least somewhat compatible with Gtk2 so there's a consistent desktop.
What should have been done, is the CSS in Gtk3 should've been designed with backward compatibility in mind, and the tooling to automate the creation of a Gtk2 theme based on a Gtk3 stylesheet.
As it is, they can't even move from 3.0 to 3.2 without breaking themes. I've no idea what the situation was for 3.4 or later, but I imagine much of the same.
Also, theming is missing a usable distribution and installation model. Currently, users are expected to simply extract an archive into a specific directory and follow any instructions that come with the theme - some include shell scripts. This is hardly "user friendly," like they claim they're attempting to make Gnome.
