Now explain to me why he retired.
You gave a good list of heroes who all suffered a great deal while pressing on... But I think there's a few simple reasons why Tirion wasn't starring in this expansion, if I may.
1.) Every expansion will have a great evil. Tirion Fordring doesn't have to be at the helm on all of the conflicts. While he is a great bastion of good, we shouldn't have to rely on him to save us every expansion.
2.) The Plaguelands is still covered in plague and Scourge. Tirion and the Crusade (many of which are Dawn) wouldn't abandon their original mission when it still isn't finished. They seem to be the type to see a mission through to the very end, when the last Scourge is dead and the last vestiges of the Cult of the Damned are wiped from the planet, and when Lordaeron is back to the way it was.
3.) Tirion's suffered to. He had his titles and life taken from him for standing by his beliefs, having to watch his entire homeland turn into a wasteland, ultimately coming to watch his son die right in front of him, in his arms even. Tirion's done a lot of good in the world for a man his age... And he's not getting any younger. Tirion might not be long for this world, and it would be fitting if he could finally have some semblance of peace in his life before it ends.
You might be right in saying Tirion isn't a character for Cataclysm, but, really, it seems as if the writing made it so that Thrall was the only character for Cataclysm. It shouldn't be a fault of Tirion that the writing that's circulating around Thrall.
All that said, I believe Tirion's capable of coming out again.
I'd say the same thing about Thrall, who seems to be wanting to take the same route Tirion has (thus far) taken, wanting to live a peaceful life now that he's had his time to shine. Thrall wants to have a family, to live in peace with his friends. Does that mean he should be killed? I wouldn't say so.