Can't help with the diff tool concept, but I built a search engine for talks, which semi-randomizes the results to force a variety in the search results.

For business talks, you can see it here: http://findlectures.com/?p=1&type1=Conference&talk_type_l2_C...

This includes the older startup school talks, plus a few good software business conferences. For the next phase, I want to highlight talks that are most unique to the category (using word lists in the transcript/captions, not video diff)