Saying that it's your opinion does not get you a free pass.

I've noticed a trend in class debates. When someone is arguing a point, but someone else brings up a good counterargument, they'll retreat and say, "Yeah, well, that's my opinion" as a kind of defense. Generally this person is someone who has figured out what freedom of speech means, but not what "opinion" means, because they won't pull out this defense against a valid opinion; they'll use it to defend an argument. While everyone is entitled to their opinions, saying that you're right because it's your opinion and it cannot be wrong does not get you a free pass. If you use the "opinion clause" on things that aren't opinions, then your statement is still up for debate, and it can, in fact, be wrong. Quite wrong, actually.

For instance, the following is an opinion. "I don't like girls." It is a subjective statement about your personal feelings. 

The following is not an opinion: "Girls are dumber than boys." It is an objective statement about reality. It doesn't explicitly say how you feel about girls; it makes a testable claim about the real world, and thus is subject to scrutiny, and ultimately, a ruling on its accuracy. 

In conclusion: An opinion is a statement about how you feel about something. If your statement ends up saying something about anything other than you, it is not an opinion, and can be proved to be wrong.

Addendum: Another thing. Even when "it's my opinion" is used on something that is an opinion, it still doesn't mean that the opinion can't be examined! Though something subjective can't be "wrong", per se, it can still be misguided. Saying "it's my opinion" doesn't mean that everyone should just accept your feelings on something and not examine where they come from or why you feel that way.