> And all income earned by a US citizen is subject to federal taxes, regardless of where in the world the income is earned.

This is just the same as many countries. All income earned is subject to tax in the country you live in, regardless of where in the world it was earned. However, this only applies when you are tax resident. So I can't see any reason why a wealth tax in other countries couldn't also apply to global wealth. There's a precedent set by their income tax already applying to global income.

The almost-unique difference in the case of the US is that it taxes its citizens even when resident outside the US (I realise there's a deduction for taxes you've paid overseas).