AFAICS tax on unearned income like pensions is what causes the most dismay, because there's no threshold. So a pensioner who married a Brit and lived in Coventry rather than Arkansas ;-) and qualifies for a modest UK pension, now is dismayed to discover that her pension is unearned income that is subject to US tax because she's still a US citizen.

I think that for the 2 years that I worked in the UK, I earned enough to get 50 pence a month when I retire :) The US tax man is welcome to all of it