I'm building a small social Q&A site. Another Q&A site that I use was recently branded by Google as a content farm and removed from Google results. I know what Wikipedia says is the definition of a content farm (low quality paid articles and spammy text across the page to catch search engines). That other site I use doesn't do those things, so there must be more to it than that. I want to make sure I don't do anything that causes Google to think my Q&A site is a content farm. What should I do, or avoid doing in designing my site layout?
A content farm intentionally puts low quality content up for the sake of placing ads (also usually by spamming the search engines with tons of content to drive in that traffic). In the case of Q&A sites that other site had to have extremely poor content to be removed by Google (which site was this as I haven't heard about it and it's possible they were punished for other reasons).
Since your site will be driven by user content you need to put measures in place to ensure the content is of decent quality. Make sure content from other websites is properly cited. Make sure external links are relevant. Don't allow the same questions to be asked repeatedly. Etc. The community moderation the StackExchange network uses helps to ensure the quality stays high. The result is less noise and great rankings.