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:

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