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.

How abstract is this though? Can I say take the main menu and do what I like with it? Can I grab the local task menu? Etc.

Rather can this be left completely to the Window Manager? I.e this is a menu, this is a tab. Now present it as you will.