How much should I unit test?

A few months ago I was debating with a friend over IM about how far one should take unit testing. He was of the opinion that one should only bother with unit tests on complex methods, and that unit testing the simple stuff was a waste of time. I pressed him on just how simple an app or a method had to be before it was too simple to bother unit testing, and he responded with the simplest example he could think of: “After all, you don’t really need to test ‘Hello, Worl!'” [sic].

That was the end of that argument.