Inherit labels in outgoing messages.
There should be an option so that when you reply to a message that has been Labeled then the outgoing reply would inherit the same Label.
I like the idea of including labels on outgoing messages. That way you can sort your sent messages (sent folder)as well for easy finding. This way I will not have to cc myself.
A related feature would be the ability to assign labels to an outgoing message, something that is particular useful when one chooses the "send and archive" option and wants to be able to later track the archived message using label organization.