The problem usually comes from people using opinions as if they were arguments. If all you're doing is going "hey I liked A and you liked B, that's great, we both have something we like!" that isn't a problem - but it also isn't how most forum posts play out. Instead, it's "hey I liked A and therefore this show is great", or, more commonly, "I hated A and therefore this show is bad". And then they are pressed for more substance, and their defense is "A is just my opinion man, you can't deny me my opinion!" - forgetting in the process that while "I hated A" absolutely is an opinion, "this show is bad" is NOT, because it implies a reasoned conclusion. Which they failed to provide, and try to substitute for with an opinion.
It's all well and good to say "I don't like this show", but that's not the same thing as going "this show is bad" or "this show has terrible writing" or whatever, all of which imply they're reasoned conclusions and therefore demand reasoned justification in the form of arguments, not opinions. It's the difference between going "I hate chocolate ice cream" and "this shop's chocolate ice cream is terrible" - they're not the same statement, because while no one can argue against you personally not liking chocolate ice cream, they absolutely can argue about the quality of a particular shop's chocolate ice cream based on standards and criteria relating to chocolate ice cream.
Don't confuse opinions and arguments, and don't try and use one as the other. And this absolutely goes both ways, of course.