How sure are you really that criticism of republicans and libertarians is not the issue?

I think that "The Non-Libertarian FAQ", although not published in LW, is popular here. If we all could debate politics on this level, we probably wouldn't need the norm against discussing politics.

I don't have a similar example for Republicans, but I guess most of them would be offended by reading LW opinions about religion.

You would in fact receive the same reaction if you told a communist that their beliefs are factually incorrect.

I haven't yet met a communist who would react to criticizing their beliefs by saying: "please give me some specific evidence". They usually react exactly the opposite way; the experimental evidence is the last think they would want to discuss; it's the great idea that matters and there is no need to learn from history, because next time it will magically work perfectly. And those are the more sane among them; the less sane will say that all evidence is just American propaganda, including the things I have seen with my own eyes as a child. (There are many communists in my country, so it is not difficult to meet enough samples.)

In fact I agree that my biggest fault was that in the OP, I said that the other party is "kind of crazy" instead of "Holds beliefs that are kind of crazy" and edited it accordingly.

Without saying which beliefs specifically you mean, this is not an improvement. Okay, I guess it is a small move towards politeness, but not towards fact-based discussion.

people generally enjoy being offended. It gives you that nice feeling of righteous indignation.

This is true in general, but this is not the main problem with your article. If you think it is, your model of LessWrong is incorrect.