It's not like it's a surprise. We have three tanking classes, and I believe both Paladins and Druids have in the past pointed out that they are both better multi-mob tanks than warriors because they can output more damage, and thus more threat (as their threat is much less static than warrior threat is, due to their lack of static threat-building abilities and their reliance on various forms of damage to build aggro) to more mobs than a warrior can. Indeed, the change that made Thunder Clap usable in defensive stance was an attempt to make warriors more viable on large packs in the new instances.
But the objection from warriors has been, and will most likely continue to be, that they are expected to make sacrifices to tank. A feral druid who wants to go out and solo up some mats for crafting or grind up some gold can swap on a new set of gear and go raise some hell with significantly improved DPS. A protection paladin with a few points in ret can go AoE farm like a baby mage in plate armor on lower level mobs (so that a level 70 tankadin could go farm elementals in Nagrand all the live long day if he or she wanted to) while the protection warrior can strap on his best DPS gear and still be far, far less effective at killing things than the other tanks, much less other warrior specs. Note that I'm not saying it can't be done. I've done it myself when I was specced protection: slap on my best crit gear and go out and bash things with my shield, hoping for a Shield Slam crit to get the damage up. It's not ideal, since while shield slam is a protection warrior's best damage instant (and is so lovely for tanking, believe you me) it gets most of its damage from your shield block value. And if you're in DPS gear, you probably don't have a lot of shield block value. So yes, it can be done, in much the same way that one can choose to walk uphill in a snowstorm in sandals.
Whenever I spec out of protection to do a PvP weekend or to try and make a big rep grind push by killing demons in Shadowmoon Valley for the Marks of Sargeras and Fel Armaments they drop, I feel as if a fifty pound weight has been removed from my back. My DPS goes way, way up. Mobs die so fast in comparison that I start to feel like a superhuman god burst free from the bonds of petty earth. I start doing crazy things like forcing my guildmates in the picture above to take me on runs where I tank with two weapons in zerk stance even though I know that it is wrong, wrong, wrong. Luckily, guilds exist to slap you on the back of the head and remind you why you have a shield. But the fact that there is such a giddy rush of release when you step out of protection spec means that the spec has some sort of fundamental flaw - it shouldn't feel like being loosed from Tartarus when you take a weekend off to PvP!
I'm going to quote a poster on the 2.3 warrior thread posted here this week: "I tried tanking too many pickup groups and got burnt out, I think. Warrior tanking feels like you aren't playing against the monsters who are trying to kill you. Warrior tanking is a demented contest, a race against your teammates, with you on a bicycle and them riding motorcycles, where they chivalrously agree to give you a ten yard head start." One of the two problems with protection warriors isn't their fault at all nor is it the fault of the spec, it's the fault of how warriors were originally designed to tank and how the game has changed since then. It's the fault of static threat generation. Most warrior threat moves generate their threat wholly independently of damage dealt. The result of this is that, after a warrior hits level 70, trains all his newest skills, he's basically done. He's not really going to see any significant increases to his threat as his gear gets better. He'll see improvements in his survivability, for the most part. More armor. More health. More defense to push crits and crushes down. But his threat really isn't going to scale the way everyone else will. As a rogue gets better and better weapons and gear, as a mage gets more and more damage, as the fury warrior gets more and more haste on his gear, the protection spec warrior gets less and less benefit to his threat from his gear while everyone else gets more and more threat from theirs.
Their threat scales. His doesn't. This is why you see protection spec warriors putting as many points in the spec as they do - there aren't a lot of 20/0/41 or 15/5/41 prot warriors. The main benefit of better gear is the ability to shift your spec away from Anticipation and Toughness, freeing up points to go further down the tree. I would consider this spec a minimal protection spec build with 46 points (and some of those talents were just chosen to get further up the tree - I'm actually not a big fan of improved disarm personally, although as always your mileage may vary and it is good against melee bosses if they can be disarmed) but you can see how going that far down protection really takes away a lot of options for a warrior who wants to actually kill anything. Needing talents like Defiance and Focused Rage to just keep abreast of the scaling of everyone else's damage means you yourself will not have the abilities that warriors use to deal damage. You will not have a hefty instant attack, you will not have a rage dump, you will not have abilities that increase your critical hit damage or inflict a stead DoT or send you into a flurry of attack power boosting crits.
Switching gear can only compensate so much for this essential flaw in the design of the class. Other tanks benefit from overhauls that have more recently made them viable. Their tanking methods are not so hobbled by static threat, they're not designed around tanking two year old content. Warriors as tanks have a great deal of viability due to their high survivability in the proper gear, using the pure avoidance of block, dodge and parry to totally mitigate incoming damage when fortunate, but their threat mechanic is antiquated. Changes to the class since have revealed this essential weakness - the minimal damage done by Revenge was increased in the same patch that gave us thunder clap in defensive in an attempt to raise warrior scalability both for soloing and for threat generation, and it is a nice start. But frankly, I'd like to see devastate get more damage as well for the same reason, so that a weapon upgrade helped scale a tanks threat up and so that a properly geared protection warrior could actually do some damage. When a warrior tanks an instance, his damage is usually on the bottom of the charts, usually only eclipsing the healer who isn't doing any damage at all. While I don't want to see protection as a spec offering a way to have main tank levels of armor and defense and doing anywhere near as much damage as a rogue, as it stands a prot tank could triple his DPS and still not even come to within a third of most DPS in a run.
Static threat doesn't scale, and a lack of gear scaling hobbles prot viability more than anything else when it comes to soloing and grinding. It's been pointed out many, many times now, but Blizzard seems to be intending to increase the viability of every spec in the game, giving players more options. But at present, warriors have one spec for tanking, one spec for PvP, and one spec for PvE DPS. Why shouldn't prot be viable in arenas? Why shouldn't fury warriors be decent tanks? Especially when it comes to prot, the warriors who have to give up much of the 'fun' aspects of the class, some sort of accomodation similar to that of the recent healing gear changes needs to be thought up, whether it is an overhaul to static threat or some new, better idea I am unlikely to imagine because, as I freely admit, the folks at Blizzard are much better at seeing the whole picture of the game than a dude who plays mostly warriors.