Actor Allusion: During Jessica's cameo in American Reunion, she announces that "I'm a lesbian now." Natasha Lyonne played a lesbian character in "But I'm a Cheerleader" the same year the original American Pie was released.
Seann William Scott (Stifler, the dude who drank semen beer, gets peed on and throws the wildest parties)
Jennifer Coolidge (Stifler's Mom, the MILF)
The Other Darrin: Stifler's little brother is played by Eli Marienthal in the first two films, but by Tad Hilgenbrink (as a high school senior) in the spin-off movie, Band Camp.
Taken Up to Eleven with Nadia who in the second movie doesn't look anything like in the first one. This might have been a means of demonstrating that Jim doesn't actually find her that hot anymore now that there's Michelle.
Sequel Gap: If you ignore the direct-to-video sequels, American Reunion is the first film with the original cast in 10 years.
Star-Making Role: Seann William Scott as Stifler. Tara Reid, Alyson Hannigan, and Eugene Levy were well-established at the time. The rest of the cast hasn't been... quite as lucky.