Great post. Very clear and concise as usual. I recently read an interesting article by Eugene Volokh on slippery slopes focused specifically on gay marriage, which you can find in pdf form here. (If you don't like pdf's, the title is "Same-Sex Marriage and Slippery Slopes.") Interestingly, he also discusses the US's first amendment as something like a Schelling point, though he doesn't use the same terminology.

In parts of Europe, they've banned Holocaust denial for years and everyone's been totally okay with it. There are also a host of other well-respected exceptions to free speech, like shouting "fire" in a crowded theater

From what Volokh says in the article, it seems that many countries aside from the US don't have as strong of protections for freedom of speech. E.g., in Canada and Sweden ministers have been prosecuted for comments condemning homosexuality.