Executive Meddling: A couple times, when the judges believed an act made a bad song choice, the performers mention that producers influenced that decision (Arcadian Broad in Season 4 with a High School Musical number, the Fiddleheads doing "Billie Jean" in Season 6).
Fan Nickname: Tends to be labelled "America's Got Singers" by those who dislike the abundance and favoritism displayed towards singing acts.
Since the success of Fighting Gravity in Season 5, every year has seen at least one notable group that uses similar light-on-a-darkened-stage effects (Team iLuminate in Season 6, two or three in Season 7 but Lightwire Theater is the most similar).
After the success of Kenichi Ebina in Season 8, there have been several more acts that do a similar miming/dancing to video thing.
In Memoriam: The final audition shown in Season 12 was preceded with a message saying that the performer had died recently (he was killed in a car accident), and that his audition was being shown with his family's blessing. The traditional memorial message was shown for him afterward.
Nick Cannon's then-wife Mariah Carey did a guest performance during Season 4's finals.
Similarly, Heidi's husband Seal was a guest judge in Season 12's Judge Cuts.
Stunt Casting: Season 12. Does the name Nick Uhas ring a bell? Big Brother fans will remember him as the guy from the notorious Season 15 who had high prospects coming in, but went out much too soon. Ironically, Nick also loses on AGT, too when he doesn't escape judge cuts, but he has a much more flavorful time to show his stuff on this show and really demonstrates his worth in spades by doing tons of cool science experiments. Strategist or just plain chemist, he's certainly got quite a lot of fans!