This place is great, but there is a bit of orthodoxy in the mindset - quite unfortunate, actually, as I don't think that fits at all with the 'hacker' mindset.
If I came here and posted a comment to the effect of "Paul Graham is a huge idiot", I would expect to get downvoted, but I will have succeeded in saying it.
Is your idea of freedom of speech "nobody gets mad at anything I say, even if my remark is formulated specifically to get them mad"?
So,
a> no, no adverse effects other than being downvoted (which is a form of suppression of speech through discouragement).
b> that's a bit of a leading idea, assuming I am referring to statements which serve no purpose other than to intentionally inflame. I'm talking about honest opinions, offered as such, which happen to go against the group orthodoxy on HN.
I wouldn't say 'getting mad' would be an issue. Actually, what I'm thinking is more in line with what you are assuming - people make the assumption that a certain line of thinking is being offered solely to be provocative, and they do not take the ideas as having been offered seriously, and rather than engage the idea, downvote as if the intent of the poster was not to seriously engage in discussion.
It seems to me like the problem isn't that there are certain things you aren't allowed to say, but that there certain things that a large group of people aren't going to even bother taking seriously. It could be because they've seen the argument before and consider it both obviously wrong and inflammatory, or it could be because they've been brainwashed / self-deluded. Regardless, that is a problem with humanity, not the hacker news community.
Most times, however, people stop after they get unfairly downvoted, and think to themselves: 'well, that's group-think for you!'
This bears repeating. People here keep saying you shouldn't care about downvotes, but you nailed it. In effect, it amounts to "silencing dissent" (when applicable).