Drk has some RNG elements (like all the new classes?).Drk has 6 abilities dedicated to regenerating Mana (Blood Weapon, Blood Price, Syphon strike, Carve & Spit, Soul Survivor).

Cross out their similarities and ignore efficiency at whats similar for a second..
Pld has a Shield, Cover, Shelltron, Divine Veil, Clemency, Tempered Will,
Drk has Sole Survivor, Scourge, Dark Mind, Blood price and Plunge? Eh... Its all rather selfish. Hell Blood price (and Sole survivor?)doesn't even really count. More importantly their "Selfish aspects" are of questionable value? A lot of attacks have been physical which puts Dark mind into question. Scourge is dps, whatever. Sole Survivor is for adds not bosses. Plunge makes you a better tank?

As far as mitigation goes their only hard advantage is Dark Mind, many skills are dedicated to making their mana work lol or doing damage. Unless Drks DPS is *that* good or Dark Mind is that useful, probably hard to justify them other the others.