For example, it's not quite the same thing to be polite as it is to be effusively welcoming

But would that be better? If one is too effusively welcoming, that might be seen as creepy and chase people away.

Now, the optimal strategy would be one that creates a true impression that one is liked, and that others would like one to remain around. (If someone is not liked, then the optimal strategy, assuming infinite resources and time, would be to persuade them to become more likeable). The question, then, is what strategy creates that impression?

I think that the karma system does a part of that; it allows a visitor to find out which behaviours are appreciated and which are not in a simple and straightforward way.

I also think politeness is not quite enough to handle frustration, which may mess up otherwise productive discussions. For example, when someone offers interesting counter-arguments, I usually don't have a problem thinking about them pretty calmly. But I've found people will frequently talk past each other?

Ah; I think that this is, strictly speaking, a seperate issue; the issue of clear communication. Paraphrasing the other person's argument is, I've found, often a helpful way of dealing with such a situation; so the handshake that you propose is a valid solution. However, the already-present meme of tabooing certain words seems to me to have the same, if not greater, benefits.

A certain degree of empathy - by which I mean, understanding what the other person is thinking - is also a useful skill to develop for this sort of situation.