Beyond my woes of ditching Progressive Enhancement for an SPA architecture, I think my biggest takeaway is: Users expect rich interfaces.
Dave Rupert talking about building a side product. Going the full stack rails way is exactly what I would have done (and have done) in the past to build an MVP. My feelings have been starting to shift to API first recently. I think this allows you to iterate more quickly on the client(s) and not be tied to an html producing backend server that you have to meticulously sprinkly javascript on to get the experience that users expect. I’d probably even start with the client first to drive out exactly what is actually needed from the backend. This is what my current team did with our product before I started and I think it has worked out well so far.