[codeofdusk]

> Taxes? Annual reporting of some such?

US citizens are required to report all worldwide income for taxation. Elsewhere, requirements to pay tax generally depend on residing in the country for some number of days per year.

> do you need to present your US passport when leaving the US and your foreign one when arriving some place?

Yes.

> Would you have to reveal at the border that you are a US citizen?

At the US border, yes. If, say, you have a European passport and you're entering Europe, you present the European one.