Cata and Wrath dungeons are a world apart. With Cata, Blizz seemed to decide that they wanted CC back, curing and interrupts should matter and things like that. There's not a whole lot of AOE tanking or "just heal through it".

With the guy you're talking about, you should cure the debuff, but the vital thing is moving. He's a "teaser" for the second boss, Ulthok. You need to move out of his green poison puddle {which he can throw anywhere, so watch your healer, too}, but not kite him so much that the puddles end up boxing you in.