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:

• Watching out for it (well duh)
• A handful of explicit norms - roughly, don't assume any country as default, and gently point out when other people do it; but keep your own national norms when they're not too stupid (e.g. no site-wide conversion to British spelling, SI units, or Lojban), though cultural translations are welcome
• Increased exposure to other countries' culture (if I can become US-centric that way, why not the reverse?)
• When talking about problems that concern a country, compare two countries (or more); this should cause you to notice you were only thinking of the US
• When you have expertise in something, check its domain of application, including country and culture; e.g. notice by "actors" you mean "Hollywood actors"

(Also, lose the "hey, this is about overcoming bias" line. Saying something is a bias and why is enough, we know biases are bad.)