Red chests contain torches, food, helms, consumables (except scrolls), and miscellaneous equipment.
Early red chests usually contain a low level torch or food. Boss chests have a high chance of paying out with a Glass, Luminous, or Infernal Torch. However, since those torches also often appear in the shops, they may be out of the pool by the time you get to the boss.
The Bomb and Bombx3 items are removed from the red chest pool if they spawn in the level as pickups. The blood drum is removed from the pool once you leave the floor with the blood shop, as the Blood Shopkeeper drops it when he is "killed." The same happens with the Glass Jaw and the Glass Shop.