Most of us have wished, at some point or another, that we could go back and change the past. Make another decision at a crucial moment in time, which perhaps would have changed the direction of our lives. Goon is a story built on this concept, and it's even better on a second or third read.
Micah gets the chance to apply the knowledge and skills of his older self to an important period in his youth. One of my favorite parts is the moment where he first takes the alternate path and stops denying his feelings for the love of his life, even though he's unaware this is what's happening, and what the dangerous consequences will be. Another favorite part is the way he deals with his bigoted mother, so clever and funny.