The tag input box does a great job of discouraging new tags, and encouraging the use of more popular tags.
Retagging is also one of the first "edit" privileges you get (at 500rep), so there are more retaggers than people able to edit questions.
There's a section of the 10k-rep tools that shows newly created tags.
I think the community user already does tag-maintenance - it occasionally retags posts, for example I recall it retagging one of my questions from using "editor" to "editors"
I asked a question here "Batch Retagging""Batch Retagging" about this very topic - basically allowing users to suggest (larger scale) tag improvements.