EUians: Anything to watch out for when your company gets acquired by a US comp?

A friend of mine works for a tech start-up in a western european country, which has just been acquired by a US company. No significant stock options are at play. My friend is just an employee, not a founder. There's always the threat that they'll make everyone redundant, but let's ignore that for now.

There are many legal and cultural differences between the US and Europe, but you don't know what you don't know. I wonder if anyone can expand on any other differences? Will the new US managers come in and expect something that's totally illegal or totally out of the question. I don't know anyone who's worked in both EU & US, so I don't know what the differences are. Has anyone been through this before and have any advice of what to look out for?

It's better for everyone if this goes smoothly, so any help anyone can give would be great.