Question: What is Hardcore mode in Red Dead Redemption?

Answer: Hardcore mode increases both the difficulty and realism of the Red Dead Redemption Single Player experience.  This mode is exclusive to the Game of the Year edition of Red Dead Redemption for both Xbox 360 and Playstation 3.  Here is a list of notable changes:

• Aiming is permanently set to Expert.
• Maximum health is reduced by half.
• Health regenerates more slowly and begins refilling after a longer delay.
• Dead-eye does not automatically fill over time. It must be earned by killing enemies or using items.
• Horses do not regenerate. If killed, you must purchase, steal, mount, or break another horse and then hitch it to a post to earn the right to call it by whistling.  Note that horse deeds are still available and unlimited, but twice as expensive. 
• Store items cost more. Sell values are lowered.
• Reward/drops/looting amounts are lowered (ammunition and money).
• Pardon letters are more rare.
• Fame and Honor gains are reduced.
• Mini-game buy-ins are lowered to match the scarcity of money.
• Warhorse deed must be purchased for $10,000 (available in any General Store).
• Deadly Assassin outfit increases the amount of Deadeye gained from kills, rather than over-time regeneration.
• Note that Hardcore is only available in the normal Red Dead Redemption campaign -- Undead Nightmare is unaffected.

Other Tips

• As with the original Single Player campaign, health regeneration in cover is higher than out of cover.  Use this to your advantage!
• Master Treasure Hunter increases the maximum stack size for all items from 5 to 10. You can now carry twice as many dead eye items at once.
• Master Survivalist allows you to loot an additional type of dead eye regeneration item from plants, increasing the number of dead eye items you can carry.