I was thinking about what you said about the first "big deal moment" in Discorld being the dragon's quote in Guards! Guards!, but I think the very first one of those was even earlier than that. From the second book: "But somehow the wrath of the gods would have been better than the sound of that voice…The voice didn’t believe in gods, which in Rincewind’s book was fair enough, but it didn’t believe in people either."
That’s such a good line and I’m pretty sure I missed it in my read-through! (Of course back then I was still trying to use quote format for everything I posted so I did way fewer and also they didn’t look as good)
I feel like there are Big Deal Moments in every single book and you find them as you read and re-read at different times – I still find things I never noticed almost fifteen years later. I highlighted the one from Guards! Guards! as one of the first because it was one of the first ones I remember, when I was reading the books for the first time. (The VERY first one I remember is this little one, from Jingo, because the first time I read the series I read them out of order, but I talked about the Guards! Guards! one first because I’m reading them in order for the re-read so I got to it first).
And sometimes you grow up and you re-read a book and a thing that wasn’t a Big Deal Moment to you when you were a kid suddenly takes up your whole world for a minute and it’s all you can think about and it’s the Biggest Big Deal Moment yet. I still don’t think I’ve run out.