1. Get our attention. You don't need bombs and explosions (although I think they help the video below,) but do let us know what you'll be discussing right away. Otherwise we'll click to something else.
2. Don't bore us, get to the point. For most web videos less is more. :15 seconds might be a little short, but 1 to 3 minutes is usually about right.
3. Tell us what to do. That's the call to action. What do you want viewers of your video to do? Make sure you tell them with a simple call to action. If your video is longer than my :15 second example, include the call to action up front AND at the end.
Of course there's more to good video, lots more, but without these three elements your video will have a tough time succeeding on the social web.