Back at the dawn of time, the debuff limit was 8. That means only 8 debuffs could be on a monster at any given time - which lead to severe restrictions when you had 40 players attacking the same thing in a raid environment. (You would either always have important debuffs being bumped off or you would have very strict raid rules which only allowed certain debuffs to be used by certain players.) And warlocks, whose DOT spells actually made up good portion of their damage done, were reduced to shadowbolting machines. The debuff limit was later doubled, and currently sits at 16, which allows for a lot more flexibility. However, recently the CMs dropped what I consider to be quite the bombshell: in the expansion, in addition to smaller raid size, the debuff limit will be increased again - to 40. Combined with 25-player raids, that's a heck of a lot of debuffs...!