Yes, I've hijacked my friend's idea. What's he gonna do? Sue me? Actually, he probably will, but never mind...
One of the worst things anyone can do when reviewing a film (apart from disagree with me, obviously) is to review a film without having seen all of it. No matter how bad the film is, if you switch it off before the credits are rolling, you lose all rights to review it.
A film critic wouldn't review a film he had not seen, or seen only part of, so why do you people on Amazon believe you can do it? If you miss the end, then you will also miss crucial parts of the story. If, for example, you missed the end of The Sixth Sense, you'd miss the twist at the end that changes the whole film. If you only watched the first half hour or so of Audition, you would forever believe it to be a romantic comedy, never to know the horrible truth. To review either of these films without knowing the full plot would be grossly unfair to them. The strange thing is, people seem perfectly happy with reviewing a film they only see the first half of or sleep through the middle of, but don't review films that they missed the start of. People seem to forget that the ending of a film can have an effect on how the whole film works as a whole.
The moral of this piece is: to avoid appearing on this website, watching any films you intend on reviewing from start to finish is a good place to start.