Challenging viewpoints, disagreeing politically, arguing — these things aren’t what the OP is talking about. He’s advocating for protecting Hate Speech, specifically (and later in the comments the original author extends that same protection to libel and slander) — which is exactly the kind of speech that has harmful, violent consequences (or in the case of libel & slander, has other negative consequences). It’s not actually hard to identify or categorize Hate Speech. There’s not actually a slippery slope in categorizing it — just as there hasn’t historically been a serious problem in dealing with Libel and Slander, which currently are NOT protected speech.
The rest — that’s protected speech. And should continue to be protected speech. Nobody is actually arguing to stop Free Speech. We just want to stop protecting those who advocate and incite violence, and hiding behind a weak understanding of Free Speech to let them get away with burning democracy down. Or, in other words — we’re not anarchists.