Theoretically they can ban you from ever entering the US again even with a valid foreign passport. In practise this doesn't seem to happen. But I guess you never know when they'll start enforcing that rule. There are also complicated tax implications - basically they treat you as if you died and tax you one time on all of your assets.

For those who are so inclined you are not permitted to possess or purchase a firearm if you have renounced US citizenship.

That sounds likely enough, but do you have a source?

edit: nevermind, it's in 18 U.S.C. § 922 : US Code - Section 922

I only knew it because that is one of the questions that you answer on a Form 4473 (the form used for background checks at gun stores). There is a whole list of questions to which you must answer in the negative, and that was one of them.

Link, with anchor, to the specific clause: http://www.law.cornell.edu/uscode/text/18/922#d_7

... presuming you're in the US.

Can't vote in the US, obviously.

Finding a job in the US would be more difficult, presumably.

If he is Canadian, there is always NAFTA.