At present, all four tanking classes can 'do something else' in a raid. Two of them can do anything that needs doing, be it DPS, heal or tank. One of those two can even DPS in either a melee or ranged role. The other two can only DPS or tank. Still, that means in any 25 man raid these classes are often expected to provide tanking if necessary (as I stated before, if a normal tank is unavailable or extra tanking is needed for a specific encounter). In a 10 man raid, however, if you already have your tanks, it can often be difficult to ask a third tank-capable player to do so due to the DPS and healing thresholds necessary to successfully complete an encounter. If you have 2 tanks, 2 healers and six DPS and the only other available potential tank is a resto druid healing your raid, it's simply inadvisable to ask him to switch unless you so completely outgear the encounter that a sing;e healer can heal it, and in that case you probably don't need a third tank anyway.
Basically, what we see happening here is that 10 mans combine the inflexibility of a 5 man run (since it is very rare that a 5 man will ask someone to off tank - all four tanking classes therefore must be capable of tanking whatever will be tanked be it trash packs of large size or a single boss) with the fight mechanics of larger, 25 man raids, especially once 10 man raids begin exploring hard mode raid content. This means that 10 man raids often have to deliberately be designed around the limitations of a 10 man group, and thus encounters have to lose complexity or stress the raid group beyond its depth of selection.
Meanwhile, at the 25 man level, raid tanking is more and more about two unrelated kinds of play itself. Trash between bosses is basically just a boring, autopilot experience for everyone involved as tanks grab as much threat as they can on as many mobs as possible and the DPS burns them down. Occasionally there will be trash pulls that require CC or discipline from the DPS such as the trash packs before XT-002 (let the tanks move the adds out of the big glowy shields please) or General Vezax, but these are infrequent, and even when there are specific mechanics (try to kill the statues at the same time or they go Supermode) they usually don't provide much in the way of real difficulty for the tanks. Either the DPS adapts or you pop some cooldowns.
Boss tanking, however, is more and more about conditionals. Some bosses hit very hard with a specific ability that requires more than one tank to eat the effect, like Meteor Fists or Auriaya's Sonic Screech. (In the case of Auriaya, on 10 man at least we just put the entire raid in front of her and split the damage that way, but on 25's since we often have four or even five tanks we just have them eat the damage and the raid stands behind her). Other bosses, like Gormok or Thorim have a specific attack that requires tank rotation to remove either by resetting or allowing it to tick off. Still other bosses just hit ridiculously hard with an ability (Sartharion with 3 drakes comes to mind) requiring a tank to use his cooldowns (and often, the cooldown abilities of his or her healers as well) to survive.
So we have a tanking game that is divided into two (and congealing into three) parts. We have five man dungeons, even heroic dungeons, wherein 1 tank per party will tank everything that is tanked, be it trash or bosses. In this milleu, even the hardest content currently available, any one of the four current tanking classes is capable and there is no significant perception of tanking inequity. Some players may prefer the Death Knight's ability to generate AoE threat or significant cooldowns (depending on their individual spec) while others might like seeing a warrior tank with their strong cooldowns and variety of unique options like Vigilance, Intervene, Heroic Throw (with a silence if talented), and Warbringer. Still others might prefer Druids or Paladins. In the end, the 5 man game is wide open and any tank can tank here just fine. More importantly, the player base is aware that any tank can tank here: there's no sense that some tanks are just leaps and bounds superior for tanking heroic Trial of the Champion, for instance.
Now, in raiding, we see a different situation. To discuss some of GC's points:
Typically one of three outcomes happens:
1) Players generally accept that tank class choice has little bearing on the fight.
2) Player suspect tank class choice has a small bearing on the fight, but it isn't usually worth the hassle of swapping out.
3) The typical way to do the fight is to swap out for the class that makes the fight much easier.
Ghostcrawler's second option, in my experience, is rare to the point of vanishing. There is no raiding guild that will hesitate to make a swap if they believe it will make a fight even 1% easier, especially if they're learning a fight for the first time. That leaves us with numbers 1 and 3 of his example.
There are a great many fights in Ulduar and Trial of the Crusader/Grand Crusader where we roll with the tanks we have instead of seeking some magical perfect tank that will make the fights easier. Part of that is, we generally have our dedicated tanks at a level of gear that makes them the clear choice for survivability and threat. Furthermore, those dedicated tanks are accustomed to the roles: they know the healers and the healers know them, there's no having to stop and explain what cooldowns which will be using or to go over what to expect in a fight. I would argue that it is easily the case that in the vast majority of cases, you'll not swap out a tank if you don't see a reason to. Inertia rules, and people will most likely want to remain in the roles they're accustomed to (with exceptions based on the occasional feeling of stagnation in a role or what have you.)
For Sartharion and for Vezax (to name just two encounters) it felt* this way with DKs. Enough* guilds seemed like they were swapping to DKs for those fights because they made the encounter just so much easier. In fact, they made a lot of fights easier, so the conventional wisdom seemed to be just tank everything with a DK. (As another example, the conventional wisdom in BC heroics and Mount Hyjal seemed to be to use a paladin tank because of their huge AE threat advantage.)
I wanted to emphasize this paragraph because it seems to me to be the crux of the matter: namely, that this is not a case where a tanking class has to have a massive advantage over others. I've now tanked Sarth+3 on both a DK and a warrior in 10's, and this is this patch, where DK's have lost a good deal of the insane cooldown flexibility they had previously. My healers stil prefer the DK over the warrior, because the DK can hold threat on both Sarth and a drake (as well as quite a few adds) and can cycle through several cooldowns to stay upright. This is despite my warrior's advantage in health and armor (my warrior is significantly better geared than my DK) and in fact, is despite the fact that the DK's advantage in both AoE threat and cooldowns is minimal at best.