Overall, I think this is a good move by Google and hopefully a lot more of the older WordPress themes will start to adapt as a result. It's great that Google are offering good resources and information to back up their latest move. There is also a good Google blog post here: http://googlewebmastercentral.blogspot.co.uk/2014/11/helping-users-find-mobile-friendly-pages.html revealing a bit of information about what they actually test for (I expect the list is not comprehensive and will grow over time).

RWD (responsive web design) is pretty much part of a standard website build nowadays, whether that is a custom build for a business or a theme. So, this should start to show up those themes that are no longer supported or are very out of date.