I think it would depend on what kind of tanks your guild has to fill the slots. Or rather I should say, if you don't have a Prot Paladin, it may not be worth the effort to tank Keleseth, not to say it wouldn't be possible. A Paladin already has a large arsenal of abilities as the previous post has highlighted, the key fact of which, is that they can be used at range.
A warrior would be damn near impossible to efficiently tank with out running the risk of a (melee + emp. lance + anything else) combo. The damage is already very spiky across the board, adding regular melee swings would not be recommended. In addition warriors have the worst ranged capabilities.
Druids fair slightly better than warriors with savage defense to reduce the melee portion of incoming damage and feral faerie fire to tag nuceli. It could work, but whether or not it's more efficient to do so is yet to be determined.
DKs are probably the only other traditional tank that are equipped to handle it. Melee/Dot in-between casts, DnD, and DC to tag nuceli. A DK probably wouldn't even need to be tank spec'd.
The key factors in choosing a traditional tank for Keleseth will be who your raid has available and the individual skill level of the persons in question.
I'm not sure how a warrior would have any problem surviving the melee swings. Furthermore, you only get meleed if you are in melee range, which is something you should not be doing during the empowered phase (since you are constantly collecting new nuclei).
Also, warriors are a highly mobile tanking class with potential to use intercepts and intervenes to cover distance. Add in the ranged weapon and they should be quite adept at picking up nuclei.
I will agree, however, that a DK is well suited to the job. Regardless, this is not a typical tanking job and you really do need to consider more than you usually would when deciding who can handle it. A warrior, with all the things I have mentioned (intervene, intercept, ranged weapon, and a few other things), should be just as capable as a paladin in this role.
I wouldn't go as far as to say that the fight prefers 3 tanks, after all the mechanics are largely the same in 10man where it's unlikely you have 3 tanks and pretty much have to use a dps to tank Keleseth. I feel that the benefit of the larger health pool of a traditional tank is pretty much neutralized by generally weaker ranged abilities for nuclei pulling and the larger amounts of damage he takes from having to go to melee for threat generation.
You still don't want to ranged tank it on 10-man. Just have one of your tanks tanking both melee mobs while the other tank handles Keleseth.
This, by the way, is also viable on 25-man. You shouldn't ever have to, but you can.