Elphaba: Oh ho. Now wait just a clock tick! I know that it is difficult for that blissful little blonde brain of yours to comprehend that someone like him could choose someone like me! But it's real, it's happened, and you can wave that ridiculous little wand of yours all you want you can't change it! He doesn't belong to you, he doesn't love you! He loves me!
A few seconds later, Fiyero literally (at least in some productions) comes swinging onto the scene, screaming "LET THE GREEN GIRL GO!". He's swinging on a vine in a cornfield. Even Glinda's confused by this.
In some productions, Fiyero is apparently short on rifles, so his bluff-threat of Glinda is done with...a broomstick.
Galinda says "Something's wrong...I didn't get my way..." before nearly fainting.
"It's SHOES, Elphaba. Let GO, already." Considering how dark the scene is, and what it's regarding... absolutely HILARIOUS!
Hilarious in Hindsight: This troper fell over laughing because she heard this line as "Let it go", and Elsa from Frozen is very similar to Elphaba, not the least being that Idina Menzel played both of them.
Galinda: Now, let's tell each other a secret that we've never told anyone before! Fiyero and I are getting married! Elphaba: He's asked you already? Galinda: (gleefully) Oh, he doesn't know yet!
From the song "What Is This Feeling?":
Both: There's been some confusion Over rooming here at Shiz: Elphaba: But of course, I'll care for Nessa. Galinda: But of course, I'll rise above it. Both: For I know that's how you'd want me to respond. (Spoken:) Yes There's been some confusion For you see, my roommate is— Galinda: Unusually and exceedingly peculiar and altogether quite impossible to describe— Elphaba: Blonde.
Also, almost the entirety of "Popular". "La la, laa-aa, laaaaa!" For certain the moment when Galinda tries to use her new wand to turn Elphaba's frock into a "pretty ball gown".
Galinda: (shakes wand) "Is this thing on?" (tosses wand aside) "Wear the frock, it's pretty!"
It's doubly hilarious when she tries to do it before the above quote.
Galinda: *ahem* (in a lilting, fairy-like voice) Ball gown. *nothing happens* *ahem* (In a decidedly LESS fairy-like voice) BALL GOWN!!!
This little gem:
Fiyero: You think I'm really stupid, don't you?
Elphaba: No, not really stupid.
From the book, when poor Boq confesses his feelings to Galinda, and Elphaba is snarking at him the whole way for sounding rehearsed. Galinda scolds her, while at the same point ripping on Boq with "Let me consider his stupid idea", while he's still sitting right there.
Nessa's hand gestures (indicating shortness) when mentioning Boq during "Dancing Through Life".
In addition to this, in Kristin Chenoweth's final performance as Glinda, she throws in a little surprise for everyone, including most of the backstage crew, in the form of a book called "How to be Popular"! She had Idina Menzel in stitches, and somehow managed to keep a straight face through it all! Can be seen here: http://www.youtube.com/watch?v=CUOiios1J6o
Glinda introducing herself to the Wizard.
Glinda: My name is Glinda, the "uh" is silent.
(Wizard gives an extremely bewildered look)
From the book, when Fiyero asks Glinda if she has children.
Wizard: Where I come from, we believe all sorts of things that aren't true. *beat* "We call it history."
When you put "Defying Gravity" through Google Translate, you get "Ignoring Gravity".
Kyle Dean Massey tells the story  of one incident while playing Fiyero where a touring production of Wicked had done something that accidentally left either oil or grease on the floor. Unfortunately during the scene where Fiyero and Elphaba run into the field after rescuing the Lion Cub, Kyle slipped, crashed, and the cage broke open displaying the mangled puppet to the audience. The funny part? The slip happened between two very specific lines, and Kyle couldn't think of anything else to say other than the scripted line.