What things that Americans do in Europe offend Europeans the most?

Read the wiki page on European Etiquette. Don't talk about politics, religion, or sex. Spend more time listening and less time talking, and follow others' lead when in social situations where you are unsure of the rules.

Just remember that most European countries Etiquette is different. Just because in France something is acceptable doesn't mean it is in just say the UK or Germany you could end up in bad situations.

Being ignorant.

Hi, I don't agree with the Vraag Meister at all. You can talk about politics, religion and sex, however, when talking about politics you better know what you are talking about. Reason is that people in Europe in general know a lot about politics. I would not bring up the Nazi area, because this can be a hot topic, and if you are not well informed about it you better shut up. Lol. I heard some Americans complain about the breakfast in Europe because it is different than here in the U.S., but actually a lot better. When talking about U.S. politics please keep in mind that Barack Obama is well liked in Europe, and George W. Bush was not liked at all. Also, almost no European supports or ever supported the war in Iraq. In general, Americans are well liked in Europe. Have a great time.

Being loud, obnoxious and ignorant. Oh and don't call football "soccer"... That's pretty much all I can think of, just be yourself and you'll find some friends regardless where you are in the world. Europeans, we are pretty friendly and open minded people with a really old culture, so try and maybe doing a little research on the country and its history before going there and you should be fine.

In Austria and London (you mention these) and many other places in Europe, you'll see that behind the older, more classical architecture and all the visual stuff that screams 'Europe!' to you, there are probably a lot of similarities to home. My advice would be that where things are different, 90% of the time they are deliberately and purposefully different. Europeans pick and choose aspects of Americana that they like, want to use, find useful. They have often rejected or discounted other things Americans might miss from home on grounds of taste, culture, aesthetics or whatever. Nowhere is this more apparent than food. Remember, Europeans are well able to eat or do what they like (certainly in wealthy Austria and London!) and are usually very knowledgeable about how Americans live. If they aren't doing stuff the American way, they are choosing not to, not failing to. Some Americans don't grasp this and whinge or whine about how XYZ is different from home. I'd advise not to :-) Enjoy the differences and have a great time!

Insulting other countries, and laughing at other Ethnic groups. That really pisses me off, and a lot of you tend to do it.