The "Hybrid Tax" is a term used to describe the baked-in reduction of the DPS of classes that can fulfill multiple roles. There is not necessarily a limiting factor that is visible in-game, nor is it a drastic reduction, and may be partially overcome with high skill. However, in general a class that can fulfill multiple roles will naturally have a lower DPS than "pure" DPS classes (Hunter, Warlock, Mage, Rogue). This hybrid tax is merely in place to maintain the popularity of pure DPS classes, since there would be a drop in their popularity if hybrid classes could produce the same DPS while having the advantage of taking on other roles as well. Again, it is not a steadfast rule or reduction but the tax is generally present and the reason why most averagely skilled hybrid DPS cannot outperform a pure DPS class in its element.