Re: units, I think we should act like TV Tropes with British vs American spelling: people use their native ones, they're encouraged to provide equivalents in the other common system but not punished for not doing so, and punished for complaining.
I think assuming familiarity with US culture is fair game, because it's everywhere. Then again, I also think assuming familiarity with thermodynamics, psychiatry, current world-impact affairs, or Belgian comics are fair game as long as there are enough keywords to look up.
Otherwise, IAWYC. It's even more frustrating because I exhibit it myself a lot (e.g. automatically assuming American attitudes to various minorities), and I've never even been to the US. And I'm reluctant to write "American" because what about the rest of the continent.
Possible cures include:
(Also, lose the "hey, this is about overcoming bias" line. Saying something is a bias and why is enough, we know biases are bad.)
Recently, I have noticed a cultural bias for the United States running through LW threads. It is perhaps to be expected of an English-language website, but for one that is about, among other things, overcoming bias, it is important to recognize one's own.
Aspects of the bias I have observed include:
I'm not the first to raise such concerns, either.
By comparison, e.g. the English Wikipedia strikes me as an example of an international English-language project that's relatively successful at recognizing and fighting systemic bias, and a whole set of template messages to mark articles with identified problems.
To quote Wikipedia itself:
The reason I haven't mentioned other obvious biases, such as gender, age, education, or First World biases, is because those (in my experience) tend to be more subtle here on LW and because I'm myself subject to some of them. However, I might cook something up on them later.