There's a limited number of debuffs that can be placed on any target - currently, the maximum is 16. This is enough where it's not usually problematic, but in a raid environment it's not difficult to hit that maximum, and when you do, the oldest debuff on the target is bumped off in favor of the new one. This becomes a bit problematic when, for example, the short-lived "shadow vulnerability" debuff left by a warlock's improved shadow bolt talent starts bumping off more important things. To fix this, Blizzard has long been working on a system that prioritizes debuffs and leaves the most important ones on, while letting the less useful ones get bumped off. However, Tseric informs us that the "basic structure" of this prioritization system will be going live in patch 1.11 - to much rejoicing.