Note to readers who might misunderstand: the above posts (parent and parent parent) are trolls. The only thing worse than dialog boxes are attention whore dialog boxes with poor writing. The user does not want to read your stupid dialog box because they should not have to read your stupid dialog box.

If you use an exclamation point, you incite the user to anger; if your message cannot be perfectly understood every time by every person with a quick skim at 30% attention, they will click a random button because they have better shit to do than read the stupid dialog box that you put there because you couldn't be bothered to come up with a sensible design.

The user has a job to do. They want to do that job. Your job is to help them do their job and make the computer disappear, not remind them that they are using yet another piece of shitty code in a shitty program on a shitty and infested piece of junk that they would destroy with a baseball bat, like that guy in Office Space, if they could do it without getting fired.