I'm pretty sure it's the context and execution that gets most people, and not simply the act.
Tirion got encased in a block of ice to die. He managed to bust through it when the Lich King killed the entire group attacking him and started to raise them as undead. The entire expansion, the Lich King was literally just toying with us to see who was worthy of being his servants, and when the time came he could just one-shot an entire zone. Tirion used the power of the Light to combat Frostmourne, resurrect us, and end Arthas. While a bit of a deus ex machina, Tirion hadn't been built up an entire expansion prior as some sort of savior in the way that Thrall was through Cata, and he wasn't picking up the pieces of a beaten warrior but throwing out the biggest Hail Mary possible.
Cataclysm was basically World of Warcraft: The Thrall Years where we saw everything from him dealing with not being Warchief anymore to being torn apart in the elemental planes to practically becoming a Dragon Aspect and blowing a hole through Deathwing. I don't think anyone can argue that it was a very Thrall-centric expansion, or at least Horde-sided. And understandably, not everyone enjoyed seeing one character get so much face time after already receiving a good amount of treatment previously. However, to then shift gears entirely and have him take a hit from Garrosh, lie in wait until we beat him, and then sneak in for the KB was not just anticlimactic but uncharacteristic of him. It's clear that it was put in just to have a reason for Thrall to be there, when there are a ton of different ways something like that could've been handled.