Not just Varian, though... Garrosh ruined a lot by assaulting the Horde's bleeding allies in Icecrown. "It was a glorious battle" my bum. And the whole gunship thing was more than part Garrosh' fault.
But I agree with you; all those cases of Horde and Alliance working together were the most awesome things in WoW... You felt like you were doing something great.
Can't agree with liking Varian, though... Even still; he's a lot better than Garrosh.