I just checked the interactive prototype and saw that most "threads" only contained few posts. I then thought about how some threads at en:WP:VP would look in Flow ā and I shuddered. The already page-long discussions would virtually get endless. Even when Flow leaves out some messages I think this could be a serious problem.
Also from a usability point-of-view I think there is plenty room for improvement: Currently a comment with two lines of text (those two lines are the real content of a comment, everything else is more or less background information, and should be treated like that) take up roughly 8 lines of space. This is four times the space necessary.
Sure, the prototype looks, nice, but I'm afraid it wouldn't be fit for real world usage. As an example I took two screenshots: The current style of the interactive prototype, and a version I quickly reduced in size:
It's only half the height while showing exactly the same information! And it could even be optimized further (e.g. the header currently takes two lines, too).