I have a site with a lot of use generated content. Users can down and upvote content of other users. Ratings can be negative. More or less SE style.
Should I add avoid indexing bad content? (ugc with a negative rating)
Here is what type of pages I will noindex.
I don't think, you should noindex all negative posts, may be someone don't like that question, and click on negative ranks, but it does not mean that page contain bad/low quality content. You should read this PDF from Google., you will get many hints, about good content.