The liability of loathsome, link-level web spam (SEM 101) has some useful data concerning what the Bing team sees as web spam -- and therefore how to avoid having your site penalized by Bing.
- Add backlinks to all UGC content. When users go onto websites that allow UGC to be created, those who use post web spam include backlink URLs to their sites, even if they don't have anything to do with the comment or, more significantly, the theme of the UGC-sponsoring site.
- Automation. Spammers often use automated techniques to repeatedly submit the same UGC post containing short, generic text and a clickable URL to their sites in every UGC-sponsoring page possible.
- Keyword stuffing. Post web spam text is often keyword stuffed.
- Massive repetition. Lots of non-relevant, poor quality, inbound links come from such pages as online guest books, forums, and blog comments.
- Large, sudden surge of new inbound links. When dozens or hundreds of inbound links suddenly appear for a new or a previously small website, such a big change can indicate link farm web spam activity. The relevance of the outbound linking sites will be a key factor in whether or not such a sudden change warrants further investigation.
- Consistent similarities between outbound linking sites. If a large number of the inbound links for a site come from sites that are very similar in design, structure, and other key characteristics, this can lead to deeper scrutiny of a website for web spam.
- Poor linking standards. A link farm will often have a large number of unrelated links on the page, or will have related links to many sites that employ other spam methods.The pages themselves are designed to maximize the number of links on them, favoring outbound links rather than original content on the page.
There is a lot more great data in the article.