At Overcoming Bias, there’s a great list on why opinions are “signs that your opinions function more to signal loyalty and ability than to estimate truth.”
My favorites are:
2. You have little interest in getting clear on what exactly is the position being argued.
4. You have little interest in digging to bigger topics behind commonly argued topics.
6. You are uncomfortable taking a position near the middle of the opinion distribution.
9. You find it easy to conclude that those who disagree with you are insincere or stupid.
10. You are reluctant to change your publicly stated positions in response to new info.
16. Your opinion doesn’t much change after talking with smart folks who know more.
19. You feel passionately about at topic, but haven’t sought out much evidence.
Let me also add that when people are expressing an opinion, usually they’re not. The best examples are done in politics. When they are expressing something, they are stating an argument. But then when you ask them to justify what they just said, the response is, “well, this is my opinion.” No it’s not! An opinion is where you are expressing some subjective point, and typically, they’re pointless to argue about them. Vanilla is my favorite ice cream flavor. That’s an opinion. It seems odd to argue about that. Obama is a good/bad president. That is an opinion. However, if you say Obama is a good/bad president because. . . or if someone asks you why you believe Obama is a good/bad president, you are now in the realm of argumentation and simply saying “well, this is my opinion” doesn’t hold.