1. Get a good site design.
2. Implement an efficient file manager ( look at fileserve,megaupload and filesonic).
3. Give preference to free users, give them good download speed. 1-2 mb. Dont make them wait 30 minutes between downloads.
4. Good upload speed from both US and EU servers.

The problem now is, if you want to be successful you'll have to implement an affiliate program as well. Without one, 99% of the uploaders wont use your site. Although without an affiliate program your host will be legally more secure.