People who say "well, that's your opinion"
I'm pretty sure this one has appeared in this list before, but it bears repeating because it's just so awful.
Of course it's my opinion. That's why it's coming out of my mouth. Am I supposed to be offering other people's opinions? "I don't buy it, but Denise Richards thinks that..." What people really mean, of course, is "that's just your opinion, and is no more valuable than anyone else's" which is, clearly, horseshit. Some opinions are better than others. Some are informed, well-reasoned, logically consistent, and properly presented, and then there's yours. Y'know, the ones based on what Jimmy Joe told you under the bleachers in sixth grade, or what you read in the Bible, or what you just pulled, fully formed like Athena, out of your asshole.