In film making, "pretentious" usually means that because of the length/ composition of a shot, the actors behavior, and the music, you can tell that the scene or moment is supposed to be deep, thought provoking, and profound, but it is not. A good example is The Thin Red Line. It's a war film with an actual plot but a lot of it is random shots of a dirty soldier staring off into the distance at sunset while a voice over of self important poetry and orchestra music plays. It's like a visual version of "I am 12 and this is deep". It's interesting also because film is one of the few mediums where "what is the creator portraying here?" is not the only question, but also its appropriate to ask "what is the creator TRYING to portray here?". When films are described as ambitious or subtle, that implies that as a viewer, you know simultaneously what you are feeling, and what the creators want you to be feeling. And when they are aiming for marks that they're just not hitting, you know that, and it comes off as unearned, ham-fisted, or pretentious.