Hard mode

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.

Differences

2-3 more rooms generate each floor.
Standard rooms with a difficulty of "1" cannot generate.
- Standard rooms with a difficulty of "1" have a reduced chance of appearing.
- Most rooms with this difficulty are very easy, making difficult rooms more common.
Rooms with a difficulty of "15" are able to generate on the second floor of chapters 1-4.
- Additionally, they can always spawn on single floor chapters.
Enemies have a base 20% chance to become Champions instead of only 5%.
Champion enemies only award pickups 33% of the time instead of 100%.
66% of Heart drops from completing rooms are removed and replaced with nothing.
- 33% of Heart drops from completing rooms are removed and replaced with nothing.
Only champion bosses will drop hearts; normal bosses will not.
Floors are more likely to have curses.
Slot Machines are half as likely to pay out.
Blood Donation Machines only give out one coin per use.
- Also applies to IV Bag.
Beggars require a minimum of 6 donations before paying out.
- This doesn't apply to Bomb Bums or Key Masters.
The Shop has a 50% chance of becoming a lower level, regardless of having store upgrades unlocked.
The drop rate of Soul Hearts from Blue Fire Places and Purple Fire Places is decreased to 10%, instead of 25%.
The drop rate of Double Hearts is decreased to ~1.69%, instead of ~4.21%.
The chance of a Micro Battery spawning in place of other types of Batteries is increased to 80%, instead of 66%.
Tinted rocks have a 50% lower chance of appearing.
Certain enemies and bosses have increased shot and movement speed.
- The Harbingers have increased charge attack speeds where applicable.
Dogma does not provide the effect of The Wafer.

Unlocks for Hard Mode

Character	Mom's Heart	All completion marks
Isaac
Magdalene
Cain
Judas
???
Eve
Samson
Azazel
Lazarus
Eden
The Lost
Lilith
Keeper
Apollyon
The Forgotten
Bethany
Jacob and Esau
All non-Tainted characters
All characters

Trivia

Before update 1.03, the reduced drop-rate affected all pickups (not just Red Hearts).
Most Hard mode unlocks are co-op babies, with the notable exceptions of Godhead, Smelter, Marrow, Mega Mush, Death Certificate and the character Bethany.
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.