edX recently commissioned a study of nearly 1,000 videos, segmenting them out by by video type and production style, and discovered this among their other findings:

  1. Shorter videos are more engaging. Engagement drops after 6 minutes.
  2. Videos with a more personal feeling are more effective than high-fidelity studio recordings.
  3. Videos in which the instructor speaks quickly and with high enthusiasm are more engaging.
  4. Khan-style tablet drawings are more engaging than power point slides.