It's natural to implicitly compare one's expectations of a given situation to the actual situation. But hard as it may be to differentiate between the two, comparing one's a priori expectations to the current situation is not a good proxy for comparing the current situation to other possible situations.
This fallacy is committed all of the time. It happens in movie ratings, college social life (in my experience people's expectations of college are way inflated), travel, etc. Basically, it occurs in any situation in which people only have access to a limited number of data points and in which expectations are liable to deviate wildly from reality. Without access to other data points, it's hard for an opinionated person to avoid making this comparison. That is why one must be vigilant about it, and ultimately admit the possibility of bias.
When evaluating something, I try to be as much of a blank slate as possible. That's why, once I know that I plan on watching a movie, I don't watch the trailers, and I don't like hearing about it beyond a simple like / not like which, candidly, I make every effort to ignore. That is why watching the top 250 is so money--I don't have to even think about what movies to watch. The other day I was watching Rosemary's Baby (#221) and for the first 15 minutes I thought it was a romantic comedy. Nope!
