Except for the Xinderoth questline, quests are mostly optional, and even in the case of Karthak, it is possible to just level past it if you know the path...
there is definitely parts where you had to complete quests in order to not be area locked. But looking back, I think im referring to the quests within Xanlin Plains realm; a golden opportunity -> cut above the rest -> thrown away -> a simple trade. If I dont have to do any of these quests in Karthak realm (already completed two [paying the dues & payback], as well as nearly finished a third [clouded sight]) then that would be great to know.