I think a lot of the worry over DRK comes from the fact that there are implied changes to the job's overall direction, but we have little information on what they actually are.
As an example, DRK was originally designed to benefit from parry procs. With the removal of parry as a scaling stat, it doesn't really make sense to keep it as a gameplay mechanic. So moves that benefit from parry procs, such as Low Blow and Reprisal were unlikely to remain in their present form. It's also worth noting that Anticipation has a very similar icon to Dark Dance. I wouldn't be surprised to see it also go, or at least be integrated into another action, such as Blood Price. You have to give up five to gain five.
We actually don't know what the status of the debuffs for PLD and DRK are at the moment. There are probably changes present, in that WAR has had the slashing debuff moved to Maim, and has lost the Path debuff. Likewise, Dragon Kick on MNK has a new debuff icon, which may indicate that they've lost INT down. With so many changes, it's hard to say what debuffs PLD and DRK have access to and which they don't. It doesn't really matter too much either way, as long as every comp can access the same debuffs.
The changes to PLD and WAR's offensive buffs are worth noting as well. Berserk looks like it will have better synergy with both infuriate and raid dps buffs, while PLD's Requiescat looks to be an additional buff on top of Fight or Flight.
DRK doesn't have a damage multiplier in its toolkit at the moment. We make up for it by DoTs, a shorter GCD through Blood Weapon, and lots of oGCDs (at least two of which we've lost). It'll be interesting to see where the difference comes from. Blood Weapon looks like it now also builds Blackblood, but will it provide a damage buff as well? Will Sole Survivor pick up its PvP trait and function as a sort of Trick Attack? Alternatively, will the damage from the two new sword moves compensate with raw potency? I can't guess which direction the devs have taken it.
DRK's gameplay hinges on your ability to gain MP, because MP is potency. Even when levelling, the difficulty depends on how many MP gaining tools you've gained compared to MP spending tools. We've gained two new blackblood ablities which seem to use Dark Arts, but we don't really know where the extra MP comes from. A lot of this depends on what the MP costs look like at lvl 70, as well.
With all the focus on PLD and magic damage, it's easy to forget that DRK is often in a similar if not worse situation in physical damage fights. With the removal of Foresight (which was never really designed for mitigating physical tankbusters on its own), we're just left with cross-role Rampart, and two base cooldowns in Shadow Wall and Living Dead, both of which are weaker than their counterparts on other tanks.
That being said, it looks like we're moving away from the concept of "damage types" for tanks. Sheltron has gone from a short recast physical mitigation move to resource-based mitigation, similar to the old Inner Beast. Likewise, WAR's Raw Intuition now costs 20 units of the new Beast gauge. Dark Mind may have been re-purposed to fill the same niche, if they gave it the ability to mitigate physical damage.
Another point worth noting is that the new defensive move looks like it mitigates physical damage, at the very least, and can be applied to yourself as well as an ally. It also reminds me visually of Celes' Runic Blade from FF6, both in terms of the aloft sword and the sparkle, as well as it's function as a sort of cover. If so, it might even give us back some MP as well. It's a pretty iconic FF ability, and it's one I've wanted to see on DRK for a while.
(The only other thing that I'm still hoping for on DRK is that we manage to get Save the Queen as a greatsword someday, as it's one of the few iconic greatsword names left. PLDs can keep Defender and Excalibur.)
On the whole, it's the sort of preview that leaves you with more questions than you have answers. But that's probably a good thing. I'm looking forward to it.