It'd be nice to be able to suggest tags for other people's questions as they are often mis-tagged.
Comments only require 50 reputation and are sufficient for suggesting tag changes. If you don't inspire the author to fix the tags, then someone else who has at least 500 reputation can subsequently fix the tags according to your comments. There isn't really any need for a system outside of comments.
Something like the way delicious.com do it would be great.