Trailers can literally make or break a film. Film-makers have 90 seconds to convince the masses to cough up the 10 bucks to see their film rather than bowling or doing heavy drugs in an abandoned parking lot.
But, what makes an amazing trailer?
Great trailers:
Should give the audience a sense of what genre they're going to see. If it's not a comedy don't make it appear like one AKA Funny People. Someone (most likely a college-aged fraternity brother) expects a raunchy Judd Apatow comedsy starring Adam Sandler. Instead he leaves with his nose runny from intense crying and the thought of his own mortality. (Thus explaining how he came to doing hard drugs in abandoned parking lots.)
People can die? Even {gasp} FUNNY PEOPLE?