Bloodthirst is a Barbarian passive skill unlocked at Level 24, which causes the Barbarian to gain life based on the amount of damage he deals, in a traditional leech-skill.
This skill is aimed to make the Barbarian less worried about health during battles by providing life leech, and should probably be taken as soon as possible if the player wishes to make tanking a breeze. It can be assumed that other skills which deal damage will also bring in health.
|| Gain 3% of all damage done as Life.
"Compassion is weakness. To press the advantage when an opponent calls for mercy is the way of true power." —Halaberd the Conqueror
No synergies with any active skills or other Barbarian passives are known.
Passive skills have had a long and storied history. When Diablo III debuted at the WWI 2008, skill trees included passive skills, much like in Diablo II. In 2010, it was announced that passive skills had been renamed traits and separated out from active skills. At the July 2011 Press Event, Blizzard announced that passive skills were once again in the game, replacing the traits system.
Bloodthirst is unique in that it was an original passive skill and is once again a passive skill, but was never a trait. For the July 2011 Press Event, it had a listed level of 1 (presumably so players could try it out from the beginning), but for the beta, it was a level 10 passive.
The beta started in September 2011, and Bloodthirst was moved to level 10. When passives were reorganized for Beta Patch 14, Bloodthirst was moved up to level 24.
 Previous Versions
 In Other Games
Players familiar with Diablo II (or WarCraft II or DotA) will recognize that this skill provides lifesteal. However, in Diablo II most skills didn't give life per hit, but there were health leeching prefixes on weapons, which could make any class gain health on hits.