Experience - whereas video is a one-way, passive experience, SL is a group one. This is a completely different dynamic, if the presentation makes use of it. If they just passively show a video, they might as well be on the web. Think of it, when done well, as comparing sitting in a movie vs at a comedy show where the audience can yell out comments that are instantly worked into the skit.
Collaboration - participants get a completely different experience when they 'see' each other. It is more involving and interactive. This gives the speaker a chance to gather instant feedback, adjusting the presentation on the fly. There is even software for SL that allows participants to give feedback at specified times via their keyboard. Also, we find that "leaders" emerge in virtual focus groups, who often bring out information from others but don't dominate as they might in a "real" focus group.
Screening - starting with a larger group, sub-groups can be created based on criteria such as beginners, those giving great feedback, gender, etc. These avatars can be instantly transported into other prepared rooms or SL environments for follow up, further Q&A, take a tour, etc.
Also, participants can click on each other's profiles and learn about each other, something many like to do.
