These are all highly subjective questions, different people will give different answers, and they’ll all be correct.
Most of them do. Demon Hunters have sigils, which I personally find interesting, warriors have charge, spell reflect and block, bears can shapeshift and owl or catweave (mostly owlweave), monks have ring of peace, teleport and rolls, and so on and so forth. Each tank have their own toolkit, which people may find fun.
Yes, all tanks are very much viable for +15 keys.
All of them. Though, if you want to solo stuff, DH is probably the strongest there.
Vengeance DH is the only tank that exists, if you ask the pugging community.