It's a systematic problem: often one wants to reward some good behavior X, but X is very hard to measure so one finds some measure Y that is highly correlated to X, and then measure Y. The problem is that once people know that it is Y that is being rewarded, they seek to do Y as cheaply as possible. And if "seeking to do Y as easily as possible" does not involve doing X, then the system breaks.
So in this example, X is producing high-quality content, and Y is getting a lot of links to you. Other examples include X,Y = (knowing the material, getting correct answers on the test) and (needing money, spending money).
Consequently, measuring Y is a good proxy for measuring not only when high levels of X means high levels of Y, but also when high levels of Y means high levels of X. Next great search engine should take that into account.
So in this example, X is producing high-quality content, and Y is getting a lot of links to you. Other examples include X,Y = (knowing the material, getting correct answers on the test) and (needing money, spending money).
This has a well-known analog in economic circles known as the "Lucas Critique" http://en.wikipedia.org/wiki/Lucas_critique