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!
Lead Encounter Designer
The Debuff Limit and You
An interesting blue post today informs us that a raiding nuisance has been resolved!