My opponent goes astray with moves 10 and 11, where he evidently thought he could get in the central break ..d5. A tactical point instead allows White to win a piece and then work on consolidating his advantage. In Class-level games, however, a piece advantage in and of itself is not an automatic win, especially if there is no glaring weakness in the position of the player who is down material. This point was made in Dan Heisman's ChessCafe article "When You're Winning, It's a Whole Different Game". By coincidence, I happened to read this just before analyzing the game, which illustrates the point nicely - I missed at least one neat way to wrap up the game (see move 31) and on move 32 missed a pinning tactic that gave back the piece. Luckily when the dust cleared I was still up two pawns in a winning endgame and went on to convert the point with careful play.
The overall lesson here is to not put the brain on automatic in the opening (instead look for more active play and to exploit opportunities, even in familiar setups), nor when winning and up material where there is still play left in the position.