I'm fuzzy as well but afaik Wave had threaded / hierarchical comments. The way they did allowed for new threads to be spun off. It better reflected how conversations happen and knowledge is collected, related to other knowledge, etc.

It also had a replay feature. So if you had someone jump in the middle of a discussion, they could rewind, replay and see how it happened. As subtle as that might seem just queues and context help understand - the knowledge and the people / group.

Those were what I remember thinking "this is The One" only to see them walk away too soon.