I don't think HN's comment voting/sorting algorithm is up for AMA style discussions. So often a single top-level reply dominates the discussion.
Actually, the sorting part is probably fine. It's hard to judge, anyway.
But for an AMA, it's not as much the sorting, but also the interface: Check any random AMA thread on Reddit and try to imagine what it would look like without all the "load more comments" links. Also try to imagine how differently the discussion would flow with all threads and replies completely unfolded.
It's a complex and interdependent balance, that, I think, the way it is handled on HN isn't particularly suited for AMA style discussion. The most interesting questions need to float to the top. But without comment folding, the two or three most interesting questions will gather a vibrant discussion and replies, causing many people to not even get past them, and the other questions won't get sorted properly because so much less people vote on them.
But, why not get the best of both worlds? Get an interesting person to do an AMA on Reddit, then invite HN to come along for the discussion? For those worried about the quality of discussion, my experience is that the more "grown up" a topic is, the more "grown up" replies you will get (and childish ones voted down), so if you get someone that's big in start-up land, you'd get a good discussion just fine, unlike what you'd get with a pop-star/TV-personality.