There's not much unique-ness when it comes to tank jobs, are there?

[Nedkel]

There is not much you could do in games like these.

Jobs performance is being calculated by the DPS they do, and deffensives are binary either they are enough to make it viable or not and thats it.

Unique gameplay does not go well with a game where only one thing is important, especially on the role that is expected a certain performance from by the community with other things taking it as "granted" like for example: pulling big, mitigate decently, bring utility, survive tank busters etc.
They cant be too different because you will end up with jobs that will struggle to do certain duty (i am looking at your HW/SB paladin in dungeons).
Thats why they gave tanks a core gameplay and add some flavours to it to be a little more diverse. It is extremely difficult to make jobs totaly different from each other, but performing the same role the same.
If someone is looking for diverse gameplay he should play DPS, since they are not tied to the strict class rules which all tanks have to meet.