Mana is just very poorly balanced for witch doctors. Pretty much any direct damage ability that costs mana is unusable because you need mana for your utility abilities.
It's hilarious because, take the wizard for example. You can spam electrocute all day every day because it's free. Then if you need to use Diamond Skin or Frost Nova to survive something.... they don't even cost resources. Your still at 100% arcane power to use for your mega nukes.
Whereas on the Witch Doctor, if you have lots of attack speed, you can actually just oom yourself solely with poison darts and not even have any mana to cast Grasp of the Dead (80% slow) or Spirit Walk (your escape). Then you wait for mana to use these abiliities, which means no attacking. If you want to use a mana damage ability like Spirit Barrage or Bats/Bears, just forget about using any utility abilities besides Big Bad Voodoo.
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The batman villians all seem to be one big joke that batman refuses to laugh at - SantaRPG