Do you speak the local language, if not start getting courses now, even if many person are fluent in english, not everyone will want to speak English. (You know the joke about Parisian who don't know English if you start talking with them in English, but would switch to a perfect English if you have a slight foreign accent when speaking french)

Expect that even basic stuff won't work the same way as you're used too. It's already an issue when moving between two European countries, even basic stuff like what you can legally find on a lease or how to get health coverage will drastically change.

In southern Europe people are pretty touchy/tactile and an America would feel sexually harrassed, Nordic Europe is the other way around with pretty distant people where an American would pass for way too touchy/friendly

Learn to ride a bike, and expect to walk way more than in the US, also remember that even though winter are milder, Europe is way far up north than the US