Currently, abilities like Ardent Flame & Lava Whip follow weapon critical strike chance, yet they are insane mana dispensers (Ex. Lava Whip)
The problem is there is absolutely no way to gain critical strike chance for such abilities, you would have to run around in medium armor, and in such case you have absolutely no chance of using ex. Lava whip as a nuke, why? Because you simply don't have any passives to support it. This makes DKs primary abilities extremely mana dispersing burst abilities. Its both PvP & PvE balance issue. Dks have the option of: Dealing low DPS or up-to-par DPS that they cannot sustain for more than 8 seconds. This puts the class immediately subpar class abilities wise. And you wonder why we all carry shields and spam deadly bash? Because its the only good DPS dispenser we have right now.
TL;DR Change Ardent Flame abilities to rely on SPELL CRIT.