My experience of the site in general has been that people are open to hearing the opinion of others as long as you are polite and make sure you don't come across as aggressive or close minded. I've bumped into a few bad apples here, but you find them everywhere sadly. In general the site's full of good apples :)

it really just depends on were? I mean on the Naruto we disgree all the time on the show and fight and bicker with each other its like a little family with your annoying bothers and sisters XD I guess you can express your opinoin *because trust me we all do* but there is going to be always someone who disagree's and will fight with you tell the end about it lol but we all love each other and always have fun with it.

If I say my opinion, which is different from other people opinion, in gentle way, people always understand it. And if not..well I don't care that much. Most of the people on fanpop are normall ones ;p

I agree with sk91 and Asvni.

I always think that if you're going to express an opinion it should be politely and with no personal attacks involved. And if someone responds in a hostile way, you should respond with the politeness and courtesy that they lack. Because rudeness solves nothing.
And I've found that my favourite spots are populated with fans who seem to share the same philosophy. :)

Though "What?! If you say something non-standard other users will eat you alive!" is true on some spots because whenever I call a canon trans girl character a girl the entire club comes yelling at me like "not a girl he's a man!!!" like can you not

I think it also depends on the opinion - how extreme it is, or how popular - and on the fandom in question.

I try to be nice about them.

It varies from club to club - some fandoms and some members are more sensitive to opinions which differ from theirs - but in general people are OK as long as you voice your opinion as politely and as positively as possible.