The "Hybrid Tax" was a term used to describe the baked-in DPS reduction of classes that could fulfill multiple roles. There was not necessarily a limiting factor that was visible in-game, nor was it a drastic reduction. However, in general, a class that could fulfill multiple roles would generally have lower DPS than "pure" DPS classes (Hunter, Warlock, Mage, Rogue), assuming equal item level and player skill. This hybrid tax was 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 option of taking on other roles as well.