I propose a new, quasi-quantifiable rule for the determination of quality in movies. I call it the Fifty-Percent Rule. Basically, if you, as a viewer, can enter a movie at the halfway point and find that anything that you missed is either deducible or irrelevant, the movie is necessarily a bad movie.
Much like Modus Tollens, though, just because the first 50% proves important does not mean that the movie is necessarily good. For example: Titanic.
I have developed this rule, as I'm sure many have, compliments of having movie channels available from my cable provider. If you have an hour to blow, sometimes you just throw on the boob-tube, which never provides you with a movie that is just starting. They are somehow always halfway done.
This has actually been a huge blessing in disguise. I can't even count the number of movies that I have watched from the halfway point, thus saving half the time, and made a determination of quality. If it was good, I am now surely motivated to go and watch the first half.
This partially-reversed way of watching has, oddly, proven to be an even richer way of experiencing movies. By knowing what's coming, but not what has been, provides a unique depth to narratives. Almost like you are watching characters fulfill their destinies of which you have perfect knowledge. There's something very tragic to it.
This applies to all forms of entertainment: books, TV shows, plays, even comics. Give it a shot. Pick up a new book and start reading from the 50% point.