Currently, you can only put a maximum of 16 debuffs (continuing negative effects) on any one target. This limit is often reached in raids, and so raid groups try and control the amount of debuffs put on a target. This is hard, because currently all debuffs are considered the same, and many common effects, such as Fireball, have a small attached debuff. If a 17th debuff lands, one of the existing debuffs--perhaps an important one--gets bumped off.
Blizzard is working on implementing a debuff priority system, so that minor debuffs do not replace greater ones. But this is obviously a large undertaking, so they are taking some time on it. So I thought I'd take a stab at what a debuff priority system should look like.
To my mind, there are four main categories of debuffs, listed below from most important to least important.
1. Controlled Enablers - These are debuffs which a player chooses to place on a target and grant some effect other than damage. Examples include Judgement of Wisdom or Curse of Elements.
2. Uncontrolled Enablers - These are effects which are randomly placed on a target, or are a side-effect of another spell, and grant an effect other than damage. Examples are effects like Vindication, the Thunderfury proc, or Shadow Weaving.
3. Controlled Damage - These are the classic damage-over-time spells, like Corruption.
4. Uncontrolled Damage - These are effects which add a bit of damage, but are more of a side-effect of another ability. The classic example is the Fireball DOT, or the warrior talent Deep Wounds.
Of course, there is a spectrum between controlled and uncontrolled damage. Some spells have both an initial component and a DOT component. Which category a spell should be in depends on how the total damage is divided.
I think that the four rough categories above are the basic elements of a debuff priority system. Controlled enablers are the most important, and should not be bumped off by spells from the lower categories. Uncontrolled enablers are the second-most important, and so on.
Controlled effects are more important than uncontrolled effects because the player deliberately chose to place them on the target. That choice should be respected. If a raid has too many controlled effects, and needs to reserve space for an uncontrolled effect, it is within their power to do so.
Enablers are more important than damage because the effects provided by the enabler are fairly unique. Damage can be compensated for from other sources. But an effect like Shadow Weaving is not easily replaced.
Combining these two rules gives the four categories above. At first glance, I think they seem pretty reasonable. There is a little fine-tuning within categories, and some ordering in the damage categories, but I think this debuff system would cover 99% of the cases in WoW.
But there are a lot of effects out there, and one important one may have slipped my mind. Is there any effect that would not fit nicely into this system?