If you don't often run raids, you may not realize that there is a limit on the number of debuffs that can be on
any of the myriad of creatures in Azeroth. A single player, or even a full party of five, isn't ever likely to
run into the debuff limitation, but the magic number is 16. When someone attempts to apply debuff number 17,
well, the oldest debuff is knocked off. Of course it could be worse - several patches ago the debuff limit was a
mere 8 - but it can still pose difficulties when working with a large group of players, due to the fact that so many
spells have a debuff component of some sort. To simplify the management of debuffs, Blizzard has been working on
system that allow buffs with a higher priority to stay on while those with a lower priority get knocked off.
However, this feature has been in development, and it's hard to say when it might appear on the live realms.
Until then, it helps to be aware of the debuffs your spells leave behind and their importance to your group. I had started to make a list with suggestions on a priority order, but when I noticed a lot of warlock spells were coming out on top, I decided that my opinion might be biased... And where these shared debuff slots are concerned, it's important to be able to step back and think about what gives the entire group the most benefit.