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?
The reason it matters, and the reason that GC is at once right and wrong about warrior tanks (ie, yes, they are not operating at a huge deficit, but no, that doesn't mean they're not operating at a deficit that is significant even if it isn't large) is because min-maxing raid leader decisions trickle down. We all remember BC five mans, yes? When groups picked paladins to tank over druids and warriors because they all 'knew' paladins were the best tanks for those instance. Paladin AoE tanking in some heroics was at the expense of healer mana and sanity in many cases, as healing the tank became an exercise in massive frustration and mana bombing that has to some degree continued to dominate the healing game, to the point where even the developers want to back away from casting the biggest heals you can on every GCD.
At present, tanking in heroics is basically all tanks tanking in this fashion. Tanking in raids is either this form of tanking or using the three or four tanks you have to pick up trash. There's very little CC and everything comes down to tanks generating aggro fast while the raid burns down targets. And in these circumstances, a warrior is always your second best choice. Now, if a warrior is what you have, then you'll most likely make use of said warrior. But if you are blessed with a surfeit of tanks in all the classes possible, you would only choose a warrior tank over one of the other classes if she or he significantly outgeared them or was significantly more skilled.
There are going to be situations where one or the other classes are going to feel easier as tanks, just because the abilities and mechanics are still different despite all of our homogenization (and we really don't want to homogenize any more).
I agree with this statement, and don't want to see any more homogenization. In an ideal world, we might have seen less of it, actually. Other tanks would not have been given the warriors tools at the strength the warrior possesses them when the warrior was not to be given those classes options at the same strength, for example.
However, right now all situations are those where one of the other classes feels easier as a tank. From a raid leader's perspective there is no fight in the game that is so difficult on healers that you would ask them "Do you need us to put a warrior in there?" In fact, in the hardest fight in the game right now, the best choice isn't a warrior or even any other class, it's another class' pet. Imagine this from the perspective of any tank who went out and worked hard to be geared enough to tank this fight being told that no, you're not good enough, we're going to use Burninsoul's big blue buddy. Furthermore, even if a warrior tank's bag of tricks makes life easier on healers, often a raid will still choose a class with faster threat generation over them on the assumption that faster threat = quicker DPS = shorter fight = easier on healers in the end.
Ghostcrawler is absolutely correct that warrior tanks are not inferior by any large degree right now. He's wrong if he thinks that means they aren't getting passed over. Warriors don't need to be the best at everything, but they do need to be the best at something.