One thing they teach at YC, and in one of pg's essays (http://www.paulgraham.com/investors.html), is to present a story instead of a list of features. That way you answer the question of "Why would I use this product?" simultaneously to answering "What does this product do?".
You can't emphasize enough to people to tell a story, and the screencast is a great crucible for whether you have a good story to tell. Screencasts aren't appreciated enough for the way they've helped people understand concepts that are a little more technical than they normally would sit still for.