Only suggestion I would have is go slower on the demo. I know you lost me very early into it switching between windows.
If you are looking for a wider audience than those who already know the context of dropbox, make a video where you lay out the case for use of dropbox using simple examples from user point of view(think a college student) and then in the demo show just the basic features. I got the feeling you tried to show too many features too quickly.
In general, I have realized it is much better to launch with something that does a few things REALLY well rather than a lot of things with little focus. When you launch with whole lot of features people assume you are competing with the big companies. When you launch small and do it well, it is easier to attract a user-base and THEN keep feeding it more advance features in form of updates.
Good luck! Looks slick from the UI.
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.