You may also want to consider allowing the user to specify whether the default is read-only mode or read/write. In similar applications such as http://join.me, the default behavior is similar to a read-only mode.
It only seems like the professional content is crowding out the home grown stuff because that's what YouTube suggests for you. If you find good content, you share it with others, regardless of its professionalism. If you want amateur videos to do well, share ones you find interesting with others.
