Does anyone know the easiest way for applying the possibility of reply to the workflow notifications from SP lists? Let’s say that recipient could just reply to the email ‘Approve’. It’d be great if it worked on all SP lists. Should it be done via exchange or mailanable list directly in sharepoint?
I have done this scenario before. It isn't exactly what you are looking for, but it will work.
I'd probably have something like a discussion board that was open to incoming email, and in the workflow have a link at the bottom of the workflow notification email for Approving, which would open a new email to the discussion board incoming email address. That incoming email discussion board list has a workflow that reads the Subject and parses out the ID, finds the task in the task list, and makes sure the user that submitted the discussion board email is the "Assigned To" user to that task. Then cross reference the ID and Assigned To to find the task and set the Outcome to Approved to complete it.
It's a little ugly, but it works all with out of the box components.