Actually, I think, one can only have a perception of the quality when watching the movie... the trailer can give an impression that you like/dislike, but the movie can - nevertheless - give you the opposite impression.
To me, it's different to talk about "the biggest quality ... a film should have" than to give a reason for choosing to watch a certain movie.
To the latter, I may reply like RTH did...it could have many different reasons...
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