If you’ve got any kind of web presence, showing up in Google’s search results is undoubtedly important to you. So there’s few things more annoying than discovering that your site simply doesn’t show up, even if you’ve specifically submitted it and done a search that includes your domain name. What gives?
Google's Australian blog has a useful summary of Google's policy on why sites get excluded, which is a good primer for anyone not already familiar with the vagaries of search engine optimisation (SEO). Leaving aside obviously illegal content, the main reason content gets dropped is because it tries too hard to appear in search results:
Common guideline violations include cloaking (writing text in such a way that it can be seen by search engines but not by users) or setting up pages/links with the sole purpose of fooling search engines and manipulating search engine results.
Conventional wisdom holds that if your site is relevant to a particular area, it will show up against a given search. That's not always true -- and Google would obviously rather that you spent money paying to appear in results next to that search anyway -- but it's always worth remembering that aggressive search optimisation techniques can have unwanted results.
Hey! My site disappeared! [Official Google Australia Blog]