For me, the decisive part in Gul'dan development was in the Audio play, the last part I think, where he want from a average power-angry loner vilain to a guy that understand being strong but alone will kill him soon enough. With the power of the tomb, he was way stronger then khadgar and Maeiv but with their combine effort, they were able to counter his attacks. He then saw how we hero would get him pretty easily with 10+ on him. He then gave part of that power back to active the portal and become a "leader" of the legion. It worked pretty well so far for him but I can't wait to kick his ass, I really hope he go down with a boom.