Online comments: Line drawn at religious, racial-sensitive issues
Here's the thing about communicating through whatever medium, be it phone, tv, youtube videos or online forums. If you cannot say the same thing you said on that medium to a live person, don't say it.
E.g. You make a racist remark on someone. Are you able to say it in front of the person you're talking about?
E.g. You accuse someone of fraud. Are you able to say it in front of the person you're talking about?
Many people get the impression that you're anonymous behind the Internet. YOU'RE NOT! You're not especially if you doing illegal stuff, such as threatening racial and religious harmony, like in example 1. You're not if you're slandering someone, like in example 2. Both of which will get you into trouble with the law.
That's how I consider if the things I write on the Internet are safe. Regardless of the medium, I can tell every person the same version of the story every single time.
Sure people might not like what you say, but at least what you say is not against the law.
Freedom of speech is only within legal boundaries.
(By the way, Paul Graham has a great essay on how to disagree with people online.)