You gave us a great feature - subtrees. But it is critical needs work. As a minimum, the following things that will make this opportunity a truly magnificent:
1) to display the status of the source subtree - whether there are changes in it , that you can PULL to yourself.
2) similarly display local state subtree relative to the source - is there a change that can be PUSHED into the subtree's source
3) an optional possibility to PUSH/PULL the source along with the general PUSH/PULL of the repository
4) AND - remember the password in the keychain for the subtree's source! it is impossible to enter it each time (and I have not one but many subtrees)