On the other hand my mother hasn't lived in the US for close to 35 years now and she still has to file a tax return and pay a few bucks every year, due to still having some financial ties with US.
Oh yea, if you have $10k at any moment in the course of the year you must report all bank accounts where the money was (i.e. 3k in one account, 5k in another, 2k in a third, report all 3). You wont hear about this rule unless you live in a so-called "tax haven". Guess why that is.
[1] This is very complex. For a single earner you can take an exemption on the first ~$80k. In most of the US that's a lot of money. Not necessarily everywhere, so depending on where you live you may hit this pretty fast. Taking the exemption makes you unable to claim certain things so you can also chose not to take the exception and use a more complex form. I have a tax guy do this for me (takes him over 3 hours to go through the paper work) and costs about $1k/year.