I think tags are useful for longer works, but when we're working with pieces this small, I don't want every branch to have three or four tags: it would be easier for me most of the time to just read the branch and find out what its content is. If we're talking ratings system and limiting it to your standard "G," "PG," "R," and "X," then I'm okay with that, but if the tags are going to get as complicated as, say, Fictionmania's system, then I could definitely see a chapter's tag cloud being longer than the chapter, and even if it doesn't get that far, it's still extra clutter.
With regards to ratings: I've long been in favor of a simple up/down system like most websites with ratings currently use, which can easily be done alongside some sort of review mechanism. The two serve different purposes, and are complimentary: one gives an immediate at-a-glance data point, and the other gives something more in-depth. Another possibility is a Facebook-style "you can only upvote" method, which eliminates trolls altogether.
