I've been watching a lot of amateur films for a local film festival, both shorts and features. The biggest mistake I think a lot of features make is that they just stuff their time with mise-en-scène
While it can be very well done and artistic, it can also just totally wreck your pacing. I'm so happy you got so much beautiful footage of crashing waves but after 5 minutes I am very very bored.
So far I would say the biggest mistake shorts make is thinking a surprising but vaguely defined twist is an adequate ending.
So many of them have interesting set up and then something UNUSUAL happens and its just over.