standing on a remote corner of a bike course and shouting 'left' for 4 hours, clapping people on (there are only so many ways to encourage, and you don't want to sound disingenuous!). if you are lucky you might get to help someone with a mechanical, otherwise it can be quite a long and lonely morning/day.
i really enjoy marshaling events, particularly small, local ones, and would recommend that anyone who regularly races should do it. you learn a lot watching 300 people attempting transition, or even riding a bike in an effective way.
it also reminds you to smile and say thanks when you are racing - you appreciate the marshals all the more when you've done it yourself.