+1 to the excellence of the prototype – it's awesome to see and be able to play with this. If a picture is worth a thousand words, I think this counts as a bona fide novella :)

I do agree with the navigability issues raised by Trevor & Brion. It's hard to get a quick overview of all active conversations. Desktop talk pages solve for this problem by providing a ToC, and the mobile web solves for it (in articles) by collapsing all sections and using level 2 headers as a de facto ToC. I can see elements of the latter method in your prototype, but the size of each conversation item and the fact that the topics are open by default both make it hard to get that much-needed quick glimpse. I also agree that it's hard to dig out the new stuff. The green badges next to "responses" and the highlighting of new items are both easy to miss and impossible to filter for. 

On a more general note, I'm still not sure I grok the board/feed distinction – I was thinking that new stuff would appear in your feed, and all stuff you follow would appear in your board, but that doesn't look to be the case. This is also a big problem I have with the concept behind LiquidThreads – if I'm already following pages on my watchlist, which is kind of my de facto feed, I really don't want to visit a separate "new messages" page unless it actually shows me a roll-up of all new messages related to those pages (as opposed to basically the full threads of three-year-old discussions, with one or two new messages buried somewhere in the middle). I'm also not sure how the watchlist is going to interact with Flow, given that, in essence, it's serving many of the user needs that Flow is trying to solve for (albeit in a weird, suboptimal way) at the moment. Have you done any thinking about how to delineate these features more clearly? 

Lastly, the feed/board distinction (if I'm understanding it correctly) seems much more useful for when all discussions (village pumps, articles, the glorious future when messaging goes cross-project...) are Flow-enabled, not just user talk. Then there's an obvious need for seeing the stuff I may be semi-passively following from various scattered discussions on one view, and having a separate view for really important stuff directed specifically at me. Having two different views for just user-to-user messaging, though, feels too heavy to me. Why not just one for now?

Anyway, a lot of good stuff to think about! Thanks again for the interactive demo, and really looking forward to seeing this evolve.