Winning an Endgame when up a pawn

I came across a game where a guy won to an easy tactic and his opponent resigned.  I noticed that had he of just castled he would have threatened to take a piece and at the same time semi-defend against the threat.  Actually I do not think there was any threat in the original position, it only became poisonous after his opponent blundered away the game the next move.  Anyway, I saw that after the castling move black could trade down up into the endgame.  I saw the game, and found that black was winning in the endgame as well!

I guess it goes to show that if you are up a pawn in the endgame it is usually a straightforward win, especially in addition to having an extra pawn one of your opponents pair of pawns are doubled.