Make People Feel Something
There are some great articles on the web about viral video and marketing. I feel like I know the technical ins-and-outs of video production, but there is a core element to any story that will make it memorable: emotion. If you can create any type of this in a viewer, they will remember. This is point number one in Thomas Baekdal's excellent web post, "7 Tricks to Viral Web Marketing", and it's super valuable on any storytelling stage.
Of course, the trick is how to create a similar emotion in as many people as possible. Different techniques will not work on everyone (what creates terror in one may be humorous to another), but I like to create situations that I know will work on me. I'm my own guinea pig and will use how I feel to try to make someone else feel the same thing. It won't always work, but using your own feelings and experiences will at least give you a yardstick. You might say it falls under the old writer's adage of "write what you know", only here it would be recreating "feelings that you feel".
Even if the intended feeling turns out to be wrong, I think that any feeling actually conveyed is better than nothing. A movie or short video that leaves that viewer empty will not only make your project forgettable, but prevent them from returning to watch your next creation. Hook 'em emotionally, and they'll return at least one more time for another serving.