Spammers would submit fake tags faster than everyday users. Since this is a social problem rather than a technical one, maybe old-school trust networks are worth considering instead.
heck they maybe already do this in a different/more hierarchical form based on a person’s Youtube graph.
They don't seem to be using that data to improve search results. I've noticed a considerable decline over the years, and Google is now definitely my 2nd or 3rd choice when I'm trying to find something.