Well... I'd watch the news the next day about the inevitable cheer and/or outrage by some.
U.S.A had an 11 trillion debt; How is that supposed to be paid back? I can imagine U.N pointing their fingers at America for debt evasion.
But, as always, they'll stop giving a shit and do nothing. So it's cool America. Just leave without any repercussion.
Secondly, I'd grow bored. Americans as a whole are a jolly bunch of people that speak Íngles. Can't say the same for Europeans, however.
Have you ever played on an EU server of any game? You are going to find - daily - people who insist on speaking their native tongue as a priority on-line. Sometimes, they don't even know English and can go through an entire game without uttering a single word, nor understanding one.
It makes for TERRIBLE teamwork and socialization.
Lastly, I'd be pissed, for all of the major games developers are located in America. I can live without games, granted, but I really, REALLY want to execute, stomp, slice, explode, shoot and incinerate people up in Gears 3. Such a fun game that will be... *Sigh*