Hard mode changes the gameplay compared to Normal in some ways to make it more difficult.
When playing in Hard mode, a small red pentagram () appears on the HUD, below the pickups.
Godhead's unlock requirements were extended in later DLCs due to the addition of more completion marks, but it can still be unlocked in the base game using the same method.
The Repentance chances for max level shops and tinted rocks were initially much more sparse. These chances were increased in update 1.7.1 a few days later.
None of the tainted characters have unlocks tied to getting all completion marks, apart from Death Certificate.