In the U.S., particularly at the WSOP, players love to jump around and go crazy when they win a big hand, even when they get lucky. It can be extremely annoying if you have just been on the end of a bad beat.
The atmosphere is also very different. In the U.S. the media is everywhere, along with fans seeking autographs.
You don't often see poker fans turning up to tournaments in Europe to cheer along their favorite player! It makes the atmosphere more exciting to see so many people watching tournaments.
From my experience of playing tournaments, I think the overall standard of players is tougher in Europe. Often I will be seated at a table of a few established pros and a couple of high-stakes online players.
In the U.S. there are loads more Internet qualifiers who aren't used to playing for huge prizes and have little live experience, so they tend to be easier to play against.
However, I will say that I think the very best tournament players in the United States are the best in the world.
The tactics of players are also slightly different in the United States. I noticed when I started playing in the U.S. that after the flop, U.S. players tend to bet out at the original pre-flop raiser quite often.
This is extremely rare in Europe, where mostly players just check to see what you do first. It was definitely difficult to adjust to at first, because it makes the standard continuation bet trickier!