My biggest concern with the interactive prototype (a concern I share with LQT) is that the individual posts/replies are way too tall. The current colon-based wiki discussion system is well suited to a high volume of short posts, which is what usually occurs on wiki talk pages. Flow looks as if it will be more oriented towards fewer but longer posts, more like an online forum.
Facebook handles this quite well, using a compact style for comments which allows many to be seen at once (although the font size is too small for our purposes).
I am worried there will be a large community backlash if discussions will take twice the screen real estate. It is already hard enough to skim posts on very-high-volume discussion pages like w:WP:ANI, and it would become quite frustrating with the proposed Flow layout.