I wonder how the tree concept works when responses are long or short. It seems to me that short would encourage more of a "ping pong" style thread which may be desirable in a tree form instead of a long-form question and reply.
It may be interesting and enjoyable to contribute to a site if there was a short length constraint placed on the reply. For sake of example, let's say it was 140 characters. That would force replies to be concise and give more of an anchor for others to jump off on in the conversation.
It could also make it more of a competition -- who can succinctly reply to the question with the most clarity?
Lastly, I think this could go a long way towards mitigating the curse of knowledge when people communicate. I see this and experience this when I participate in technical forums. It's easy to use jargon or advanced topics that the person may or may not understand. The tree approach gives a simple way for a follow-up question around the topic.
You may even consider tying this into the "five whys" which is a common questioning approach