Here at home, we wouldn’t speak with food in our mouths (hopefully), nor for that matter would we chew with mouth wide open. This sentiment is how the Japanese perceive laughing without covering your mouth. Laughter that exposes your teeth is considered horse-like and impolite, no matter how funny the joke was.
Americans think nothing about hopping into the back seat of a cab, but this action is seen as particularly rude in Australia, New Zealand, parts of Ireland, Scotland, and the Netherlands (but then again, they also drive on the wrong side of the road). Cabbies in these countries may see you as being quite egalitarian if you decide not to sit in the front seat. Let’s hope they smell better than our drivers.
Ok, this may be a semi-universal thing, but in Turkey and South Korea, walking around with one hand in your pocket is considered arrogant. Not since Alanis Morisette sang about giving high fives with her free hand has the one-pocketed-hand seemed non-douchebaggy.