A lot! It is structurally the same game but there's significant tweaks to every section. Just off the top of my head (some of this will be repeating the store entry bullet points):
- A myriad balance changes for individual weapons, powers, etc.
- Better rules for on-foot combat.
- Alt rules for Critical Failures, Failing Forwards and Exceptional Success in chapter 1.
- A "Dramatic Finish" rule to make defeating enemies narratively spectacular.
- An expanded alt rules chapter, I believe custom weapons and corruption were already mentioned but those are the standouts among the new stuff.
- A jillion more premades.
- The setting! ft. Cool Fungus Facts!