Witch-Doctor from what little I seen is a evolving class. In the early game his summons are king and can make the normal playthrough an easy stroll. After Midgame his summons can be more of a detriment (Vampiric enemies can become unbeatable if you have a minion... even indestructible ones feed them) and their usefullness seems to go down, however they can shift their game to range.
Pets always become a liability in high difficulty games, it seems. Their willingness to attack stuff off screen gets you into trouble sometimes, even on normal. The Monk pet seems super aggressive even compared to the WD's zombie dogs or companions.
I played a pet-based WD on normal all the way up to the middle of Act 2, and switched when it seemed like my Zombie Dogs were dying in every single fight. So I tried a "close combat" WD build. It actually quasi-worked as a tank for my group believe it or not. But I felt like I wasn't getting anything done, so after trying several builds of ranged WD damage I came to.....
Acid Cloud. Oh Acid Cloud. With 5 stacks of Soul Harvest on, the damage skyrockets above the life total of anything you're fighting. With a Level 28's mana pool, you can drop 5 of those things before you go OOM, which is enough to liquefy ANY groups of monsters even when playing with 3 people. I went from only getting lucky massacre bonuses rarely to taking them AND all Mighty Blow bonuses from my party members every time. The downside is you go OOM very quickly but with Poison Dart and the mana recovery rune, you can get back enough to puke on some more people. Right now I have the Gargantuan following me around because I simply can't find the time to use another attack skill when there's so much AWESOME SPEWAGE that needs to happen.
So yeah. While it may just be the normal mode glass-cannon syndrome, it feels pretty broke. But I think it will remain effective in higher difficulties because it's a complete build.
1. Poison Dart + Spined Dart
2. Grasp of the Dead + Groping Eels or Rain of Corpses
3. Soul Harvest + Swallow Your Soul, Soul Siphon or Vengeful Spirit
4. Acid Cloud + Acid Rain
The rest of the skills are open for choice. I prefer Spirit Walk + Honored Guest for the mana regen. But Horrify + Phobia, or Gargantuan are both decent choices too. Like I said, other direct damage spells for the WD seem to take way too long compared to the massive aoe instant damage of Acid Cloud.
For Passives, Spiritual Attunement, Bad Medicine and Rush of Essence. With this set up, you're really only concerned about two and a half things: regenning mana and casting Acid Rain. You worry about Soul Harvest when you want that psychotic damage boost.
My usual tactic is to run up to a pack of enemies and use Soul Siphon to get 5 stacks if possible. (Which effectively doubles my Int and therefore my damage, at this level. It comes at a price though, I've died a couple times trying to run into range to get them.) Then I drop Grasp of the Dead and take 3 or 4 steps back. Then I unleash the Acid Cloud until everything is dead. Because of Bad Medicine, anything caught by the Acid Cloud is weakened, which helps both me and teammates if they're in the middle of the pack. If there's still enemies alive once I run out of mana, I stand in place and shoot Poison Dart to get it back or refresh my Soul Harvest counters if I can. Rinse and repeat.
This is probably the most efficient and easy way to play WD. Having tried a close combat build with Exploding Frogs, Vampire Bats and the like.....it works, but compared to 17 kill Mighty Blows and 51+ Massacres in a group setting....one is definitely quicker and killier. There's probably a way better set up for boss killing, but for pure AoE damage output I think this might be unbeatable.