Aside from unnecessary nudity and sex, nothing kills a movie more than excessive profanities. If every other word is a curse, then the screenwriter is truly lacking in vocabulary to express whatever the character is feeling or the ability to create a character based on cliches and stereotypes.
One movie that stands out in my mind as an offender is The Blair Witch Project. I remember cringing along with my poor mother every time a character opened their mouth. Every curse uttered felt uncomfortable and out of place. I'll even admit that the Saw franchise suffers from this from time to time, mostly when characters are angry or try to appear threatening to their enemy. While the actions and story pull you in, hearing Eric Matthews yell at Jigsaw or Amanda never fails to kill the mood for example. Forgive me for the expletives, but I want to give a clearer example of what I mean. The scene pops into my head with Eric in Saw II is when he gets trapped by Amanda and he yells at her with a line that kinda goes like, "I'll fxxkin' kill you, you stupid bitch!" repeated over and over until the credits roll. I know Eric has high temper gauge, but there are other ways to portray this without silly or easy go to lines like that.
Yes, people tend to curse when they're angry or trying to sound tough, but it usually comes off awkwardly and disrupts the flow of the story and sometimes the character who delivered it. Writers shouldn't depend so heavily on profanity to create a prevalent character personality or express emotion, especially anger or intimidation.