Light. Spent too many years working at an airline and never knowing till the last minute if we would get on the plane -- never mind our luggage. So it was always carry-on. Since trips were rarely longer than a week, we got by. The hardest part was having to dress (suit and tie for me) to get on the plane when we really never needed to dress that way for the rest of the trip. And now that bags cost $25+ each, there's more incentive to pack light, though at least we don't have to worry about more-formal clothing.
The one item I absolutely make sure is packed and carried on (and off!) is a spare set of eyeglasses. If anything happens to the pair I'm wearing, I'm in a world of hurt, and I'd hate to miss out on what there was to see -- because I couldn't see.