A nice solution to avoid namespace collisions while allowing users to create their own shortening URLs is to use their username as a subdomain (e.g., bcjordan.gethopper.com/meaningfulname).

Also:

* accept file and submissions by email: subject = optional shortener key, body/attachment = content

* slap a sign up with Facebook button on it

And one more thing... http://www.youtube.com/watch?v=PEgk2v6KntY