Warriors still lack the paladin's AoE threat, while paladins have equal single target threat (indeed, paladins at this point gear almost identically to warrors, have a shield slam equivalent and even use the same tanking itemized weapons) and survivability, while paladins have a tanking buff in Blessing of Sanctuary that no min-maxing raid leader will want to do without. Warriors lack the high health of a bear tank or the speed of threat generation, are roughly equivalent in AoE tanking capacity, and a prot warrior simply still cannot match the bear's ability to switch to cat for DPS purposes if the druid is not needed for tanking. Warriors are also behind DK's in AoE threat (due to the DK's excellent use of diseases + Death and Decay), roughly equivalent in survivability thanks to a DK's wide assortment of cooldowns depending on spec (My blood tank uses Icebound Fortitude, Mark of Blood, Vampiric Blood, and Rune Tap quite frequently while tanking) and again a DK is superior to a warrior in her or his ability to step into a DPS role when not needed as a tank.
So what, you might rightly ask? Warriors are generally number two in everything, making them the best all purpose generalist tanks in the game. They can do AoE tanking, single target threat, have great survivability, DPS is greatly improved from BC, why does it matter if some other tanking class is slightly better in one or two aspects of the tanking game?