Quick one for new tanks, with numbers from the official 13.4 skill tables. The single target taunts (Barbarian Taunt, Paladin Persuasion, Death Knight Provocation, Warden Wave of Aggression) all generate the same aggression per level: 7,500 / 15,000 / 25,000 / 45,000 / 75,000. Level 5 is 10 times level 1, and going from 3 to 4 almost doubles it. The AoE versions (Warcry, Illumination, Death Call, Warden's Fury) go 15,000 / 35,000 / 80,000 / 140,000 from level 1 to 4. Level 3 to 4 is a big jump
