people who¡¯ve been playing the wow powerleveling game since the original content will tell you that working with a 15 debuff limit especially for raids was a pain in the ass a lot of times. Certain abilities that were not as important would kick off the ones that were needed for a certain encounter. Thankfully, in the TBC expansion they raised it to 40. WoW¡¯s Lead Encounter Designer Daelo made a post in the official forums giving us more good news about debuff limits.

We changed how the game handles debuffs (ie negative state effects on targets) on creatures with Wrath of the Lich King. The old hard cap of 40 debuffs on a target no longer effectively exists. You can now apply way more debuffs to a target without them dropping off before their duration expires. The default WoW UI will not normally display all these debuffs, but they really are still there! source

Certainly this is great news for raiders especially since the current classes got new abilities and a new class that produces a lot of debuffs was introduced to the game. We probably never noticed the need to have more than 40 debuffs at one time but it¡¯s nice to know that we can really go wild without worrying about a debuff limit.