I don't really have much to add on this topic in general, but as someone who's played 4/5 tank classes on WoW, I feel the need to step in here and tell you that you have no idea what you're talking about. WoW tanks are not "clones," as you put it--each has a distinct playstyle and each has different strengths, which have in fact suffered similar balance issues. If you need proof of that, ask how many DKs solo tanked Ra-den back when he was current, then compare to the number of paladins or monks. WoW has a lot of problems, which is why I left it to come here, but a lack of variety in tank design is not one of them. Indeed, its tank design is possibly the thing I most sorely miss here so far...
For the record:
Druids have avoidance-based active mitigation with some self-healing on top of it. Also very high passive mitigation.
Monks have some minor avoidance, but their theme is much more reactive self-healing through purifying their stagger, putting up Guard at appropriate times, and using Expel Harm if they get low--they're significantly more active than Druids, and most similar to Death Knights in their design. Also have very strong cds. My personal favorite tank.
Paladin have passive block that accounts for almost nothing, and then active mitigation in the form of Shield of the Righteous which reduces physical damage by a flat percentage. Also has emergency self-healing, and some passive healing. Option to spec for an even more active rotation with Seraphim, or a more passive one with Holy Shield.
Warriors have passive block, but also active mitigation which gives short-duration bursts of 100% block chance, as well as the ability to "critical block" for twice the amount of a normal block. Almost no self-healing, but high passive damage reduction to compensate.
Death Knights are another extremely reactive, active-mitigation based tank. High hp, lower passive mitigation, and self-heals that scale well with damage taken. Self-healing also triggers a physical absorb shield. Some strong anti-magic cooldowns--mainly anti-magic shell--but hardly a "magic tank." My personal second favorite tank.
When I say DK and monk are self-healing--in a raid setting, I would consistently do 40-60% of my own healing on these two, not even counter purified stagger dot on monk. I don't think you can honestly call any of these clones. Does having different design cause imbalances? Yes, definitely, but I think the variety is worth the price of that overall.
As far as FFXIV tank balance goes, I obviously need to wait until 60 to see for sure, but even at the level I'm at, I can already kind of understand where the sentiments I've seen in this thread are coming from. It's a pity, since I find the class to have much more interesting flavor than Warrior, and yet mechanically it seems in every way lacking in that comparison. Have yet to try Dark Knight.