MWI, Aumann's Agreement Theorem, Great Filter concerns for existential risk, anthropic arguments in general, Bayes's Theorem in the non-finite case. But even these are not in general high priority issues for rationality. I think it is fair to say that most of the important ideas can have bumpersticker size statements. But, the level of unpacking may be so large from the bumpersticker forms that they only reason the bumpersticker form seems to do anything useful is just illusion of transparency.

Off the top of my head, the first thing that comes to mind is: supergoals and how to assess them. Second: the process of figuring out how to parse a true utility function from a fake utility function.

Rationality is --or should be -- for regular people, and very few regular people need to worry about the curvature of the universe in an average day.

Requiring rationality to be restricted to an aversion to edge-cases limits its usefulness to the point of being almost entirely without value.

To relate this more directly: that flat-spacetime thing is very relevant to understanding how "something" can come from "nothing". Which touches on how we all got here -- a very important, existentially speaking, question. One that can have an impact on even the 'ordinary' person's 'average day'. After all; if it turns out there's no reason for anyone to believe in a God, then many of the things many people do or say on a daily basis become... extraneous at best.

Furthermore: one of the things that instrumental rationality as an approach needs to have in its "toolkit" is the ability to deeply examine thoughts, ideas, and events in advance and from those examinations create heuristics ("rules of thumb") that enable us to make better decisions. That requires the use of sometimes very 'technical' turns of phrase. It's simply unavoidable.

That gets all the more true when you're trying to convey a very precise thought about a very nuanced topic. The thing is, regardless of where one looks in life there are more levels of complexity than we normally pay attention to. But that doesn't make those levels of complexity irrelevant; it just means that we abstract that complexity away in our 'average' lives. Enter said heuristics.

Part of instrumental rationality as an approach, I believe, is the notion of at least occassionally breaking down into their constituent parts the various forms of complexity we usually ignore, in order to try to come up with better abstractions with which to ignore said complexity when it shouldn't be a focus of our attention. I've gotten in "trouble" here on lesswrong for making similar statements before, however -- (though to add nuance that was more about whether generalizations are appropriate in a given 'depth' of conversation.)

[V]ery few regular people need to worry about the curvature of the universe in an average day.

So far.

I'm not being sarcastic. Sometimes writing in a way that's easy for other people to understand is just hard. Speaking for myself, normally when my own comments aren't clear it's because I've spent as much time as I'm willing to spend on writing a comment trying to come up with clearer ways to convey my idea, not because it feels gross or because I'm not trying. (For example, I rewrote that last sentence at least 4 times and it's still pretty clunky.) This seems to come as second nature to some people, the rest of us have to struggle a bit.

None of this is intended to detract from your point. Clearer writing is better.

Forgive me, but this seems like a little bit of an overreaction. You're the only one who's called me out for writing style (although I have no trouble believing that others have thought the same thing and not said it). Frankly, I don't comment much, but when I do, my comments tend to be reasonably highly rated.

The incomprehensible-to-outsiders thing strikes me as a reach. LW by all appearances is growing rapidly without noticeable worsening in the quality of discourse or community, which is a remarkable accomplishment. When outsiders do complain about LW being unapproachable, it's not because of people like me writing long sentences. It's because of jargon, a lot of shared background that takes time to catch up on, and the novelty of some of the ideas.

I've already said I will make a reasonable effort to do better. So, respectfully, with that promise, I think I've shouldered enough responsibility for improving colloquy around here for the time being.

(Because I don't know how well in control of my tone I am, I want to clarify that I appreciate your feedback on my commenting style, and I very much do not want to come across as annoyed or snippy.)