There are these groups in (god, let me think, that windy instance...), where there are Adepts that heal like 40-50% of a mob's hpbar back in no time. If you get two and not CC one of them, no amount of dpsing will get the other one down (which also heals himself, of course).
On pulling, if you keep them on the spot they patrol, your magical dps will be useless. Spells will get redirected.
This is a normal instance and only one group.
So, yes, CC and careful pulling is what this will be about, I'm sure.
And.. I really do love that.. Brings so much back to instancing..
Edit : Vortex Pinnacle is the instance I meant there.