Polite conversation/small talk with strangers. If we have nothing to gain from talking to one another, can we not have polite silence?
The convention whereby everyone is entitled to their opinion, when the important corollary is forgotten, that one must strive to hold the best opinions they can. "It's my opinion" does not mean it is correct, and if that opinion leads to the harm of myself or others I will try and convince you to get rid of it. Similarly, you do not get to force your opinion on others, while saying of objections, "That's just your opinion." See: childish 'movement atheists'.
Along the same lines, and before the word loses all meaning, the idea that objectivity is both a great thing at all times and in all areas, and that objectivity is best achieved by clashing opposing opinions.
Edit: sexism, racism, ableism, &c. &c. &., obviously. This absurd notion that some people have that political correctness is this evil thing that's trying to ruin the fun of white dudes, dudes that [redacted before I get carried away].