On math.stackexchange you can edit the tags by hovering to the right of the last tag. A button labeled edit tags comes up. I was asked to add a tag to a question I asked and this did not work. I had to click the edit button and work on the post in the edit window. I believe this site will require a lot of tag editing, so making it easy is a good idea. Maybe it is because I don't have enough rep on this site yet. Please institute the math.stackexchange behavior.
This is a privilege unlocked at 10000 reputation for graduated sites (2000 in public beta, 1000 in private beta). It has absolutely nothing to do with math.SE or specific sites.