One thing I've heard from several Europeans I've met (who are currently living in the US, either for work or school) is that Americans have a reputation for being very friendly but making a lot of empty social promises.

One European I met at my local watering hole was a programmer and a very cool guy, so we made plans to get together for beers and nerd talk. He mentioned the whole thing about social promises and said that he's had tons of cases where an American would express an intention to hang out at some later date and then flake out.

I insisted that I totally intended to call him up the following weekend and set something up, but of course I totally forgot, and by the time I remembered it had been an awkward amount of time, so I never did call.