Hi TuringTest, thanks for your comment! Contrary to your past tense, Flow continues to be in active development, and continues to be deployed to new use cases, most recently a new user help forum on French Wikipedia, and a technical support forum on Catalan Wikipedia. Since the only way to roll out a system like this is to replace existing use of wiki pages, we're proceeding conservatively to test it out in social spaces where people want to try a new approach, and improving it in partnership with real users in those venues.
It's true that talk pages, being ordinary wiki pages, support "making your own workflow". I love the Douglas Engelbart reference, though I doubt Engelbart would have remained content with talk pages for very long. The lack of a discrete identity for separate comments makes it impossible to selectively monitor conversations you're participating in (you literally have to use diffs to know what's going on), or to show comments outside of the context of the page they were added to. This is a pretty tough set of constraints to work with. At the same time, you're absolutely right that a modern system can't simply emulate patterns used by web forums or commenting systems like this one.
Like wiki pages, Flow posts have their own revision history. Flow-enabled pages have a wiki-style header. Each thread has a summary which can be community-edited. Threads can be collapsed and un-collapsed by anyone. All actions are logged. In short, wiki-style principles and ideas are implemented throughout the system. At the same time, we believe that as we add modern capabilities like tagging, we can replace some of the convoluted workflows that are necessary in wikitext. Already, Flow adds capabilities missing from talk pages -- notifications for individual replies, watching specific threads (rather than a whole page), in-place responses, etc. More to come.