Currently each cleric that casts CH on a tank gains some amount of hate, we'll say 1000. Thus after 10 CH's, that cleric has 10,000 hate. The warrior is busy building hate this entire time, and we'll say he has 15,000 hate. Thus the mob stays on the warrior. Which is how it should be.
The problem occurs when multiple clerics are healing the same warrior. Two clerics alternating CH's can heal a warrior 20 times in that same time period, which should generate 20,000 hate and draw the mob off the tank...but it doesn't, because that hate is split between two clerics.
Does this make sense? Imagine you are a powerful being, and you're trying to kill a pesky metal thing that keeps poking you in the foot. You smash him good, and he's nearly dead, yet before you can finish him off...he gets healed. So you smash him again, and again he is nearly dead, but just like last time, before you can finish him off...he gets healed. I don't know about you, but it wouldn't take me long to realize this pesky metal thing isn't going to die so easy, and I'd kill some other pesky thing. Does it have to be a cleric? No.
New AII suggest this: instead of giving 1000 hate to the cleric every time CH lands, give 800 hate to the cleric, and subtract 200 hate from the warrior. In the original example, one warrior gained 15,000 hate while one cleric gained 10,000. With this new system, the warrior would gain 15,000-2,000=13,000, while the cleric gained 8,000 - keeping the same 5,000 hate difference as before. If two clerics CH'd 10 times on a warrior they'd get 8,000 hate each, instead of 10,000. The warrior would generate 15,000 and lose 2*10*200=4,000, for a final value of 11,000 hate. Again the mob does not jump on a cleric...but it will jump on someone else.
Where?I don't recommend this system for existing content, but it is certainly an option for new. A mob with this AI doesn't need to have the single target damage output that current mobs have to be challenging. The challenge will exist because the mob is more intelligent. It is now to the point where it's essentially impossible to make a MT mob that is challenging (yet beatable) without additional AE effects, at which point it's really the AE effects that make it challenging, not the MTing. This AI brings back challenge, regardless of what AE is used.
As a TankTanks will approach this by trying to taunt and gain aggro, with a finger hovering over defensive disciplines at all times. When the mob turns, pop that disc and hope for fast assist healing. If your clerics are good, you'll get it. If you are good, you'll survive that long. Every tank gets to participate, instead of just the first couple of tanks on the MT list. Everyone matters and everyone plays. Suddenly, tanks stack just fine on raids.
As a ClericClerics will want to assist heal, and not with CH - clerics will want to use the fastest heals possible. This means higher mana output, but higher mana output is a good thing. It means fights can be fast and exciting without having to be 15 minutes long. It means fights are more exciting and fun, and unlike CH chains, fights will be interesting and something to look forward to. Not mindless work.