I could just let you read this article to know what I'm getting at, but those who can't be bothered, to paraphrase, a sacred cow in fandom is anything that the fans feel is beyond critique. In other words, bashing it will automatically get every self-righteous member of the community to come to it's aid.

Now this is not a thread about listing which games you consider sacred cows, I am more interested in discussing the phenomena that creates this mentality. While I placed this in the general gaming forum, feel free to list other examples if it will help clarify your reasoning.