"You can't recycle video and expect to create a good online user experience."
Jakob Nielsen in his latest Alertbox:
"While I'll surely have many more guidelines later, for now the main guideline for producing website video is to keep it short. Typically, Web videos should be less than a minute long.
"A related guideline is to avoid using video if the content doesn't take advantage of the medium's dynamic nature. This doesn't mean incessant use of pans, zooms, and fades to add artificial movement. It does mean that it's better to use video for things that move or otherwise work better on film than they would as a combination of photos and text.
"Finally, recognize that Web users are easily distracted, and keep distracting elements out of the frame of your shots. If there's a road sign in the video, for example, users will try to read it and will thus miss some of the main content."
Finally, here's an interview I did with Jakob a while back:"Creating The Loyal Customer."