Apparently some of the early television shows were essentially “filmed radio" (I know I have heard this quote a number of times, but of course right now I don’t seem to be able to find a link on google). In other words instead of developing a format native to the new medium, things got started with an essentially literal adaptation. The equivalent in online education is putting entire long lectures up on the web. Don’t get me wrong, I love some of the stuff that’s on Academic Earth, but the value of delivering content this way is only slightly above distributing it via DVD.
A more web native approach would seem to be to develop microchunked videos. That would allow for easy linking with each microchunk having its own URL (yes, deep linking to videos is possible but not very elegant). Comments could focus on just that section as opposed to an entire lecture, which might cover a great many things (check out for instance the many different concepts covered in just a single physics lecture by MIT’s excellent Walter Lewin). Ideally, there would be multiple versions for each microchunk by different instructors offering alternative explanations.
Now it turns out that there are a bunch of sites pursuing this approach, mostly aimed at K12. In the US there are Brightstorm, Educator, Thinkwell and Guaranteach. In Germany there is Sofatutor. Oddly though, so far none of these sites appear to have taken off. Compete and Quantcast have none of them at more than 30K visitors per month even though a bunch of them have been around for over a year.
My working theory right now is that none of the sites have cracked the code on how to have both good SEO and spread virally. Ideally, a student doing their homework and using google to look up a question would find a video, find it compelling and spread it via email/IM/etc to other students in the class. I believe that whoever gets this right first will really take off.