MOOCs and other online classes give us an opportunity to learn what works in presentations. For example, I've proposed studies of the effect of playback speed or including a talking head in a presentation on comprehension, retention and engagement.
I'm not sure how these studies would turn out (though I have my suspicions), but I am sure that talking heads are important in coversation.
My certainty is based on a single anecdotal experience (of no scientific value). Some friends and I meet for a weekly podcast bull-session on technology (Yet another tech show). We do the podcast using a Google Hangout.
As you see here, video thumbnails of all the participants are shown under a larger head-shot of the person who is speaking. (Switching is automatic). But, check the thumbnails and you see that the second from the left is blank. That is because one of us had a slow Internet connection that night and could not stream video. We could hear what he said, but not see his face.
If you had asked me before we did that podcast whether not seeing him while he spoke would matter, I would have said "no" -- figuring that the audio conveyed all the information in a technical discussion.
If you ask me now, my answer is "yes, for sure." It had nothing to do with information or content. We understood what our invisible colleague was saying and he participated fully, but it felt weird.
Talking to disembodied people on the phone feels natural because there is no point of reference. But, in a mixed conversation, where you can see some, but not all of the participants, the difference is palpable. I don't know if it is because of subtle clues as to who gets the floor next, some primitive fear or something else, but it feels more natural to talk with someone you can see.