It would be extremely convenient to be able to automatically add an email address to an existing filter as follows:

1. User has some folders setup already
2. User drags a new email from friend@somewhere.com into folder "friends"
3. Small dialog pops up to ask if you would like to add this email address to the filter(s) that send to the "friends" folder.

To make this simpler, you could have a default filter per folder, and this dialog would simply add the dragged email's address to that filter. It would then be simpler to check if this email is already part of the filter.

A small improvement but it would make setting up and keeping things organised very much easier.