Hero Mastery Taunts are a method to flaunt your dedication to the Heroes you own.
Whenever you issue an in-game taunt using a hero who has reached level 15 or higher that hero will now display unique "mastery" visual effects overhead. The hero will also play their taunt animation, as well as the voice line you selected for their loadout. Mastery taunts won’t stop at hero level 15, either. Their visual effects will continue to evolve across several tiers, becoming more epic as your hero reaches new heights in the progression system.
"When we originally added master skins to Heroes of the Storm, they were intended to help you flaunt your depth of experience and time investment with your favorite heroes. However, when we re-evaluated master skins, we realized that there were several places where we could make improvements. Master skins created situations where players had to choose between unlocking a hero or unlocking a master skin, which was a battle that heroes won far more often. Additionally, once players purchased a master skin, they’d then have to choose whether to use it or another skin in-game. This meant they couldn’t visually display their prowess with a hero in-game if there was a non-master skin they liked better. These issues combined resulted in far fewer players unlocking and using master skins in-game than we had expected. Most players who had access to master skins did not unlock them, and most players who did unlock them would not primarily use that skin during their matches. In Heroes 2.0, we decided to move to a new system that highlights your prowess with your favorite heroes so that you don’t have to make those difficult decisions."