You keep your credits and items and unlocked heroes.
It resets achievements, hero levels, upgrades, and impossible difficulty is disabled. Note that if you save up 10k credits and reset, you can immediately get the two achievements associated with credits (get 5k and spend 10k).
I was hoping to do an impossible run from a fresh start but oh well.
If a dev reads this please consider a game reset keeping hero levels and impossible difficulty unlocked. Not a big deal though. Absolutely my favorite game this year. God bless.