The druid steps forward and says "I have Battle Res, possibly the best ability in game for guilds when it comes to learning new content. I can also innervate, and I'm really good at switching to dps when I don't have to tank. If we ever get to an extreme fight that requires a lot of healers, I can respec if we need it."
The Prot warrior should also be able to switch to dps, and they come with Battle Shout and Commanding Shout, two very solid buffs. No, they can't heal.
The Paladin steps forward and says "I can cover one of your 3-4 primary Paladin Blessings. I'm still the best AoE tank for large pulls, like those spiders you see scampering around down there. I have many auras which are useful to stack in the raid, I have wipe prevention in the form of DI, and I have plenty of other tricks like BoP or BoF, or Lay Hands. Oh, and I can respec healing too if the raid really needs it for a fight."
If the paladin is so much better at AE that you take him instead of a warrior, then we have a problem. Yes, pallys are able to heal. That part is totally valid.
The Death Knight steps forward and says "I know I'm new, but I'm the WotLK favored child and you know it. I'm the best Magic boss tank, and you know there'll be fights tailored to me. And I bring just as many tricks as the Paladin, only mine are cooler cause they involve ghouls and exploding corpses."
Again, the "best Magic boss" shouldn't be to the extent that you call the raid when she can't come. There will be no fights tailored to a particular class. Sorry. Believe me, we don't have to come up with a gimmick to encourage death knight raiders. Northrend is infested with them already. Ghouls are cool though.
The Prot Warrior steps forward and says "I can Disarm...some mobs, not quite sure yet if it'll hit bosses and how many. And there's an ability that let's me put a buff like Pain suppression on people, which..maybe will come in handy, depending on the bosses we find? And I have a single-target 10% threat reducer."
You Sunder, you Demo Shout, Battle Shout, Challenging Shout. You might Piercing Howl depending on the encounter. You Execute. You Intervene. See where I'm going...? If you are really winning or losing fights based on whether your tank can Innervate, then we've probably made the content too challenging. The difference in skill (or gear) between a potential warrior and druid tank should have a much bigger effect on the outcome.
The situations we've had before have been more like "The paladin can tank all of Heroic Shattered Halls with no CC" or "The druid can do nearly double your dps while tanking" or "The warrior can achieve passive crushing immunity and has the only ability that can handle Shear." Those are egregious barriers to having 4 viable tanking classes, which is why we're trying to fix them.