Title pretty much sums it; it would be nice if you could have an option to have a tag auto-assign if the text of an email subject matched a tag already in the user database.

Example: I have a tag for "Invoices" - receive an email called "Outstanding Invoices On Your Account" - when you select the email it is already tagged Invoices.

While this can be done manually by adding an auto-assign for each tag, that is quite tedious.