3. Add watermark
Nice idea. would be glad to have that, at least when you have this as an option during upload.
4. Use Gmail attachment's upload method (show the upload bar and stay on the same page - without refreshing the page)
Would be nice to have, or at least some kind of message when the upload fails, because of execution time limit.
[quota]6. Users can keep track of what they have uploaded (a new page in their profile)[/quota]
Is a nice idea to keep the overview.
8. Group / User permission - administrator can set a limit quota or maximum upload size for each group
This is a feature I am really looking for. I have at the moment the problem that I can not give different max upload sizes to different users/groups.