I really think one of the things that Flowdock nailed was their threading implementation - it took a little bit of getting used to, but not much, and allowed contextual conversation without feeling like the conversation was buried in a thread somewhere (as this implementation does). For people interested in the thread, you could focus on just that, but for everyone else, it just acted like a normal chat room.

Considering how long Slack has been promising threading, this seems really half baked and poorly thought out.