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.
definitely yes. You can avoid crawling of bad content also. Google also provide crawling options based on url parameters , specific path( based on robots.txt)