I participated as a speaker at two Q&A sessions today at the (this year virtual) SIGGRAPH conference. Each was half an hour long, and each brought together a group of people who had given technical talks that were thematically related.
In one case, the moderator started out by having everyone summarize their research. This turned out to use up the entire 30 minutes, so there was no time for questions. It all sort of felt like a waste of time.
In the other case, the moderator asked great questions that got us all speaking to the same themes from different points of view. It was completely awesome.
So it seems that even when conferences go virtual, some things are the same. It mostly comes down to having a good moderator.