After Mark threatens Sarah, what does John do? He throws Mark to the floor and gets ready to break his arm. Given the fact that Mark gets Easily Forgiven at the end of the movie, this is the closest thing Mark gets to a punishment for being a Jerkass. This makes this moment all the more satisfying.
It's way more satisfying in the book where, after all the hell Mark put everyone through, and sending his thugs to not only attack John and Sam, but sends them to attack innocent girls and kidnap Sarah, John FINALLY decides he's had enough and proceeds to utterly curbstomp the entire football team: Tossing them for miles, knocking them the hell out with single hits, utterly terrifying them with his strength and then, in the showdown with Mark, John beats up about 6 members of the team, most of whom are bigger than him, and then pwns Mark. The cherry on top? Sarah slaps Mark for being a dick and then John forces Mark and his gang to not only keep quiet about what happened but to stop being a-holes to everyone. For those who have seen the movie, this is sweet, sweet justice.
The true form of Bernie Kosar.
Four's defeat of the Mogadorian Commander.
John charging through a burning house to save Sarah and Mark's dogs in spite of how easily this could get him noticed by the Mogs.
John realises he can talk to animals and uses this to turn the Mog beast against it's masters which helps them win the colossal final battle.