It's more about the opinion relative to the overall context of the thread. As an example, if you try to defend or contextualise Facebook (the company's) actions in a thread about how much they suck, you'll get downvoted.

The same opinion in a different thread will probably just be ignored, because people self-select into threads based on what they care about, and the FB dislikers won't be so concentrated.

Nastiness and "can't you just" statements tend to get downvoted regardless of context though.