The Hard Mode has the following features:
- Autosaves are disabled, and manual saving costs 4 tinderboxes
- Sanity dropping to zero results in death
- Less oil and tinderboxes throughout the levels
- Monsters are faster, spot the player more easily, deal more damage and stay around for longer
- There is no danger music when the monsters are near.
So in summary: the environments are harsher, the monsters more unforgiving, insanity is deadly, and death is final – unless you pay a toll.