Now that the new PvP mode, Seize, has been out for a couple of days, here is the general impression I've gathered on the performance of the different classes. Primarily, this is an evaluation of how the classes fare at level 60, moreso than their direct impact to success in the new mode which relies mostly on human factors than class compositions in DF.

Tanks
Paladin
Paladin’s role as a lockdown tank remains mostly the same in the new PvP mode. Touting a spammable GCD stun with the longest duration of all melees (4s vs 3s of other tanks, 2s of melee dps), a 1s oGCD silence, and a vulnerability debuff in full swing, paladins work well at locking down priority targets and allowing your party to burst them down. Additionally, shield bash and cover allows for you to peel for the healer, allowing them to free-cast without interruptions when being swarmed by melees. Divine veil, clemency, stoneskin, testudo, and tank LB round off the survival utility provided by the paladin, allowing for healers to survive a melee LB if used appropriately. Despite its utility, paladins’ own damage potential has also been boosted with the addition of royal authority, allowing for an unexpected large of amount of burst, coupled with flight or fight, full swing, spirits within, and circle of scorn. Hallowed ground, despite its long cooldown, allows you to charge into large groups with little risk for either stun-duty or potential burst, assuming you back out before it expires. However, sword oath loses a lot of its auto-attack potential due to the fast hectic nature of people running around in PvP, while being in shield oath sacrifices a lot of the damage potential of the paladin. Overall, it’s a very versatile class depending on how you intend to play it.

Warrior
Probably the strongest tank of the bunch at the moment, being capable of winning any 1v1 fight on the battlefield. Berserk + Full swing into double fell cleaves pretty much guarantees a kill, while being able to use holmgang / brutal swing to keep the target in place while you cleave them or your party bursts them down. Storm's eye's slashing resistance and healing debuff makes it even easier to burst down high priority targets. Mythril tempest is an amazing skill for locking down a crowded group of defenders at a node, and can also be used as a peeling tool for healers. Thrill of war and tank LB allow for healers to survive melee LB if used quickly enough, while an oGCD stun also allows for interruption of LBs even in the middle of a GCD combo. Additionally, the ability to quickly switch to defiance + equilibrium allows you to gain an upper-hand in duels, or to provide you with some more HP buffer as you run away from death. However, despite their high damage and lockdown potential, warriors struggle a bit in the mobility department, and are more easily kited than other melee jobs. The lack of a gap closer means that they’ll be relying on using sprint + enliven / deliverance equilibrium when chasing down or keeping up with sprinting targets.

Dark Knight
I’ve been seeing a lot of derp dark knights in PvP, but the class on its own offers a lot of potential. Dark arts + souleater offers a strong, consistent damage combo (mp permitting), while DA carve and spit offers a large amount of oGCD burst. Abyssal drain, salted earth, and dark passenger provide a lot of pseudo-ranged AoE tools, but admittedly AoE damage is not very strong in itself in PvP due to its lack of burst, and the prevalence of AoE heals. However, the strongest tool in the dark knight’s arsenal is undoubtedly plunge, being a gap closer with 200 potency on only a 30s cooldown. Coupled with the consistent damage from DA souleaters, dark knights provides the utility of the tank classes (3s oGCD stun, tank LB, tar pit 40% AoE heavy, carnal chill AoE 10% damage down) on a mobile bruiser chassis. Additionally, having access to delirium and dark mind allows for the dark knights to deal with magic classes (read: summoners) effectively, using plunge + sprint + enliven for additional chasing potential. Living dead is also extremely powerful, as even if you don’t get healed back to life you’re given unrestricted movement while under its effect, effectively allowing you to get “revenge kills” even if you lose a duel.