Designated Villain: Ron, Heather's fiancee in American Reunion is undoubtedly a Jerkass but aside from suggesting a wife swap (with the consent of the wives involved) the only really 'villainous' thing he does is get angry over his girlfriend falling for and then actively cheating on him with her old flame and threaten to sue that (larger, more muscular) old flame if he assaults Ron.
Ensemble Darkhorse: Stifler went from being a minor antagonist in the first movie to one of the protagonists in the third.
That beer... Steve Stifler usually feels the brunt of these.
In the second movie, he gets pissed on from a balcony, and tastes it, thinking it is champagne.
In the third movie, the ring scene.
In the new movie, he eats out a fat chick.
Straw Man Has a Point: In Reunion the guys promptly tell Stifler he's pretty much always been a horrible friend to them. Steven snaps back they often don't treat him like a friend, and brings up the fact they pointedly didn't invite him to get together with them before the reunion. They reply they did so because they knew he would be, well, Stifler, but he makes a fair point.