I'm an American and moved from the US to Portugal a year ago, though I still spend a lot of time in the US. Here's what my biggest challenges have been:

- Family and friends are now an 8 hour flight and 6 time zones away. Staying connected is difficult, particularly given American rat race schedules. I don't have children, so this is a challenge but manageable.

- The visa -- among the easiest to get for Americans with no family ties to Europe -- took 5 months and TONS of paperwork. I had to lease an apartment for an entire year in order to APPLY for the visa. In total, it cost about $40k to get set up there. Most middle-class Americans don't have that laying around.

- The language/culture barrier is real, even in a place with lots of English speakers. I'm diligent about attending lessons and studying, but it takes time. Learning the culture is even harder and I quite regularly make a fool of myself in public.

- Above all, I've gained an appreciation for how optimized for convenience life in the US is. I dry my clothes outdoors on a line just like everyone else. This alone is a nonstarter for most Americans I know.