Mechanics
Permanent death
When you die in a roguelike you have to restart from the beginning. Other games sometime have a optional mode like this with names such as 'Iron man' or 'Hardcore'. In a roguelike it is the default and the save game does only exist so the player can resume playing where he was the last time. When the player die the save game is removed.
Procedurally generated environments
The shape of the world is generated for each new game. The player can not memorize good locations or places to avoid.
Reference