I agree with the comment about discussions ahead of time.

I think you could get people to look at slides during the presentation so that they can annotate it. I think people would generally be reluctant to make comments in real-time that would end up living online forever. I think the slides will be the high value, low cost item to collect.

Here are two other thoughts:

*) One angle could be to have people post constructive feedback in real-time, rather than having discussions.

*) Following up on Larry's comment, we could have the speakers optionally create presentations that not constrained to the time length of the presentation. That is, they could add the pointers, and other useful information for the discussion.

At the high-level (conference), it would be cool to have a contest (or a set of people that we seed with cameras) to capture the essence of the conference. This could be images, video, etc. We could award a prize.