List them. Here, I'll even give you a hand with a couple of them.
Jaster Mereel: Originally put forward as Boba Fett's "real name" in the Tales collection of short stories. When it was relegated to S-Canon by the Episode II retcon, it was no longer associated with being Fett's real name. It was kept as an alias he used in certain situations. When the character of Jaster Mereel was invented as Jango's mentor, it was an all new character, and didn't override the G-Canon retcon of Boba Fett's "real name".
The account of The Clone Wars in The Thrawn Trilogy: Kept as an S-Canon rumor thanks to the "Certain Point of View" logic applied to a lot of Star Wars "facts". Seen from outside, not knowing of Order 66, a lot of people would see the Clones as going insane.
Now, you said "A Lot of stuff." What else besides those retcons to S-CANON was retconned by the Prequels?
off the top of my head
The big one is obviuosly
"Anakin is meant to DESTROY the sith and bring balance to the force"
then we got all of Boba Fett and Jango Fetts back stories
The Entire bounty hunter triology is non canon due to the events during the Clone Wars
The Katana Fleet doesn't exist
Luuke Skywalker doesn't exist
Cloning as described in Thrawn Triology doesn't exist
The Nohgris meeting with Darth Vader never happens
again just off the top of my head.