John definitely a great post! I feel you made some very valid points though I am going to slip in to critical mode, which is probably entirely unfair to you.
One of the items that I think you should’ve mentioned would be page load time which I suppose may fit under mobile usability. Page load time is a relative thing we must consider the overall load time of the page but also the render time of the key content that is being ranked. Consider a search engine as a corporation that needs to consider their return on investment. If you are calling the page with 1.6 MB of information this could become ineffective considering the number of pages on the Internet and the amount of data that would need to be crawled and correlated. So when we talk about page load I think it’s important to consider having your most critical content render before other assets on the page render. This means no render blocking CSS or JavaScript and that images are optimized. I would consider these very very important factors due to the exponential growth of data quantity, it almost necessitates that search engines consider the load sequence above all else.
I also think negative factors not being present are a larger part of the future – when you look at the best practices released for Google developers you’ll notice things like interstitial’s getting quite a bit of negative attention, and I am sure that these prevalent negative ranking factors will be working to adjust search positioning in the near future.
Lastly I will mention rich media and specifically video. The growth of video consumption on the Internet is outstripping the consumption of any other data type by a very large magnitude and this is not gone unnoticed by the engines, whose job after all is to serve the content that users most want to look at.