If you buy a house, things are going to be complicated.
  If you have a workplace pension from your job, you're probably screwed. State pensions have screwed people too.
  If you invest in stocks, or mutual funds, you'll pay more in tax preparation than you'll make.
  If you set up your own company and are not paid as an employee but as the owner, you will be screwed.
  Rent out a property, screwed.
  Fail to declare every account that you're a signatory on? Prepare to pay up to 125% of the contents. 
  If you're children are beneficiaries of a trust set up by their non-american grand-parents? Totally screwed.

Almost every American expat I've spoken to has had a shock when they find they totally didn't understand their commitments. The only ones who didn't were employees of big banks/hedge funds whose employers paid a big 4 accountant to take care of everything.