After being pleasantly surprised by the movie Easy A, I tried to come up with a before-and-after movie scoring system that would take surprise enjoyment and bitter disappointment into account.
Before seeing a movie, rate it out of five (no half scores) on how much you expect to enjoy it. Afterwards, rate it again on how much you did enjoy it. The final score is your after score squared divided by the before score. This means movies get a bonus if you go in with low expectations but are pleasantly surprised, and that you'll get a very low rating for being disappointed.
The disadvantage of the system, of course, is that it still gives a pretty big bonus to the low-expectations movies. The best score you can get for something you expected to be great is "5", but you get almost the same for a 1-before-2-after movie.
Mokalus of Borg
PS - So this might not be quite right yet.
PPS - But I think movies that exceed expectations should get extra credit.