My second is a Witch Doctor. I'm on the fence about him. Some of his weapons seem extremely ineffective. The Corpse Spiders just plain sucks. (even with the runes I've unlocked for it) When you get Plague of Toads's exploding frogs, they can really do some damage, but their effectiveness drops of quickly the further enemies are away. By quickly, I mean if you aren't right on top of your enemies, start dividing the damage by at least 4. Even Firebomb has diminished effectiveness. Poison Dart is the only very effective primary.
For the secondary, Firebats (Dire Bats specifically) are awesome! I can clear out mobs en-mass with it. It's quite effective at heavily weakening elites too. Grasp of the Dead is good too, but the other two (Haunt and Locust Swarm) are DoTs and are relatively slow and ineffective IMO as you can kill the same mobs easier / faster using other methods. Locust Swarm can be okay when fighting bosses with lots of minions as you can DoT them and return your attention to the boss, but even in that case, Dire Firebats are still better as they kill the mobs quicker getting them out of the way.
For Defensive, I only find Zombie Dogs really useful thus far. Horrify can help save you if you get in trouble. Spirit Walk and Hex (specifically Hex) is not useful. Granted I'm only around 22-24 and haven't unlocked many of their runes, but thus far they are trash.
Terror: Soul Harvest is a nice spell. I use it a lot. Sacrifice is devastating, but the fact that your Zombie Dogs have a 60 second cooldown, it makes it useful only every once in a while. It only works with dogs you cast. If they appear by other means, they do not explode with Sacrifice. I think Soul Harvest ends up being more effective in the long run. Mass Confusion will probably be the go to Terror spell as I'm sure it will save your life a more than a few times when things get tough.
Decay: I just use Zombie Charger and send him rampaging though a crowd of mobs.
As for Voodoo, I only have Gargantuan. I'm a fan and he is always up.
I haven't really checked out the passive very much. I believe I'm using Jungle Fortitude.
As I noted, I haven't gotten to far into playing him and most of the time I've been playing my Wizard so I haven't fully figured out the Witch Doctor yet.