Looking at the "board and feed" model in Flow Portal/User to User Discussions, I wonder if the decision to "bubble" active topics to the top of the board is a sound one.
Reordering conversations misses a feature of the old talk pages, which is stability. If topics are ordered according to their creation time (just like a blog) instead of their activity, it's easy to know where a particular conversation is located with respect to all others, and use spatial reasoning and "muscle memory" to remind what you have read and what is on your pending list. You don't need to bubble up the entire topic in order to find updates; the feed takes care of that, providing quick access to any active conversation. With the board-and-feed model, reordering the board is redundant, and even harmful.
You also miss any possibility to have a Table of Contents with a summary of not-archived conversations, a really useful feature that is not present in the Flow prototype (see "substantial knowledge base" comment below). Flow seems to assume that conversations that are not active are not important and can be archived, but that's simply not true of Wikipedia talk pages; inactive conversations can contain knowledge and suggestions that can be acted upon, so they should be stored for reference, in an easy-to-retrieve way that doesn't require searching but mere browsing.
I understand the appeal of "infinite scroll" metaphor for boards, but that can be achieved with static boards ordered by creation time. Instead of modelling Flow boards after bulletin boards at internet forums, I find the blog model a better fit for the way conversations are handled at Wikipedia.