What it ultimately comes down to is the difference between a necessary ability and one that is useful but not necessary. You also need to take iconic roles into account. I doubt many would support giving warriors Blessing of Kings, Blessing of Sanctuary, Lay on Hands, the paladin system of Auras, or what have you. The ability to die without taking equipment damage via Divine Intervention? How much, exactly, is Charge worth and if it's so pivotal to tanking why are you rolling a paladin to tank instead of a warrior? How do we keep classes compelling and interesting while giving them the tools to do the same job?
This is the real difficulty with class balance and design for shared roles. It's not just one for tanks, either: healers and DPS also struggle with overlap vs. unique abilities. A lot of classes would kill for Fan of Knives. Chain Heal spawned a legion of different AoE heals, some of which surpassed it for a while. Every so often the 'give X Bloodlust' debate starts up. Everyone wants Blessing of Kings. (Except paladins, who are bloody tired of being told to use it. Get some drums, they mutter.) Homogenization isn't a curse to be avoided but it can make gameplay feel boring if taken too far. In general, I play a warrior because I enjoy the class feel as much as anything else. Cribbing too many abilities from other tanks or DPS would just feel wrong. (Of course, it's possible to change those abilities so that they feel right... I doubt many paladins hate Divine Storm just because it's basically Whirlwind.)
Likewise, I miss Death Grip terribly when I'm not on my DK but I don't want it to get shared around, it's so iconic that it would weaken the class if someone else could do it. You really don't want Beacon of Light given to druids, or Feign Death copied over to shamans. Some abilities should stay relatively unique even if it makes balance more difficult, which does seem to be something the designers keep in mind. With Cataclysm coming I'm hopeful new, fun talents with unique aspects will be coming to all talent trees across the game, helping distinguish Tank A from Tank B more. Homogenization is fine when it's absolutely necessary (if DK's just can't keep up with other tanks in survivability, or shamans can't keep up with AoE damage in raids compared to other melee, for instance) but the difficulty is in doing it without feeling like you copy pasted some other class into yours.