That's what I'm saying. "opinion" is a legitimate cause for disagreement. This is hardly a formal setting, either. Read the other articles, you'll see. calling someone ignorant because they disagree with you, however, is MUCH less legitimate -- not to mention extremely rude.
What I think you mean, but didn't say, is that you disagreed with them. Which is fine. But saying that their opinion is ignorant or illegitimate is hardly appropriate. Further, repeating over and over that someone who disagrees with you MUST be missing some information is also quite impolite at best (and yes, you did that, every single time I ventured to say that Big Fish isn't a good movie -- apparently the only reason someone who isn't clearly insane can disagree with you is if they aren't qualified to form a valid opinion on the subject).
by the way, "Normal" and "mainstream" are rarely perfectly interchangeable, actually. "Normal" is a much, much more psychically loaded term. Not just in this context, but in nearly every context I can think of. Just for future reference.