I don't really mind Tirion dying, but I'm a bit surprised everyone just stood there and watched it happen. It's not like he was killed immediately. First Gul'dan summoned a demon and then he got killed. With some waiting in between.
So let's see, we have Jaina and Mekkatorque on the Alliance, and Sylvanas and Thrall on the Horde. That's four fighters with long-range combat capabilities. Why did nobody think to shoot Gul'dan or blast him with a spell? It seemed to me like Tirion could've been saved if they really tried.
Also, why did Tirion even go off on his own anyway? Did he really think just him and the Argent Crusade could take down the Legion?