These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Nadia's changing/masturbation scene in American Pie.
The girl-on-girl teasing in American Pie 2.
Everybody really does remember the stripper(s) when it comes to American Wedding.
Jim trying to get a drunk, topless teenage girl back into her house without her parents finding out in American Reunion.
Broken Aesop: If the moral of the first film is that it's not that big of a deal to lose your virginity during high school than the first movie counts, what with all four characters getting laid at the end.
The Catch Phrase Catches On: The term "MILF" existed long before American Pie came out, but the movie is the reason it entered everyone's vocabulary.
Germans Love David Hasselhoff: Ironically, considering that the film has the word "American" in its title, American Reunion did pretty poorly in the US, not even managing to make back its budget, but earned the better part of $200 million outside of its home country.
Hilarious in Hindsight: In Wedding, Stifler (pretending to be a super-polite guy) says "I love old people". This becomes literally true when he accidentally has sex with Jim's grandmother.
That beer... Stifler usually gets 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.
The fourth movie reverses this by having him take a shit in the Jerk Jock posse's beer cooler.
Strawman Has a Point: In Reunion the guys promptly tell Stifler he's pretty much always been a horrible friend to them. Stifler 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.
The Woobie: Jim and Finch have their moments in the original: Jim for the webcam incident, and Finch for the Laxative Prank. Jim's prom night manages to be saved when he loses his virginity to Michelle, who just happens to be his future wife, and Finch gets revenge on Stifler by banging his mother, and then going on to do it two more times in the sequels.
Jim more so throughout the whole series. The poor guy can never catch a break.
Sherman really did act like a jerk to the guys when he made it appear he slept with a girl before they did, but you have to feel sympathy for him in the sequel when he's still a virgin and discovers his Sherminator act doesn't work. It's fixed at the end of the movie when he hooks up with Nadia.
Jerkass Woobie: Stifler in Reunion when the guys give him a Reason You Suck Speech by telling him he's trapped in the past because things will never be the way they were in high school, and that they didn't bother to catch up with him about the reunion because they knew Stifler would find a way to ruin it. Bonus points because Stifler was aware they were intentionally avoiding him the entire time, but acted like his normal self not to show it.