I often want to make a point that depends on the other person knowing X.

I can actually speak to the reverse of this. People, particularly in the rationalist / tech community, are constantly asking me if I know about extremely basic concepts. I attribute this to how I don't fit the stereotype of what a rationalist / tech person looks like. It's very frustrating to me to always get tailored versions of conversations that don't match my actual knowledge level. It might be fine if I could quickly answer, "Yes, I know what that is," to people's questions, but the problem is it seems that people simply avoid certain subjects all together or dumb it down so much it's unrecognizable. So, one of the reasons I enjoy online discussion is because I know I'm receiving what all the other site visitors are receiving.