The goal is to keep stacking as many damage multipliers as possible while clearing levels and mobs. The two primary means for this are Gruesome Feast and Soul Harvest. Soul Harvest grants up to a 650% damage increase assuming you hit the max number of enemies. With a 15 second cooldown and 30 second duration, it can be on almost constantly (while you keep moving through enemies). Gruesome feast stacks on top of this by giving up to a 50% boost to Intelligence. But how to get get enough health globes to keep the party going? That is why the primary damage skills (Cloud of Bats and Phantasm) are both "cyclone" type effects that work best when you are running around close to enemy mobs. Vermin adds another 20% to your firebats, which should lead to a fairly potent circle of pain around you (Phantasm only needs to be clicked every 5 seconds to add another 40% constant weapon damage around you. Finally, we have another 20% universal damage boost from Pierce the Veil. The goal is to maximize damage output and keep grabbing globes (which will help keep your mana up).
However, a Barbarian the Witch Doctor is not, so some distractions are in order. Enter the Damage diverting lifelink Dogs as well as the Gargantuan. I am on the fence about the correct rune for Gargantuan, but since the build is optimized to tear through regular mobs and levels, I am leaning towards Restless Giant, which should act as a basic meat shield normally, and transform into a useful wingman when you run into bosses or elites. Speaking of bosses, that is where the final skill comes in. Poison Dart? I know, it seems underwhelming, but bear with me. For the most part, this is a pretty mana hungry build. While cycloning through groups of enemies, the constant globe grabbing plus a partial mana refill from Swallow your soul every 15 seconds I am betting that it is sustainable (bear in mind, you can optimize gear to boost your pool/regen). But what about a single boss fight where you don't have piles of globes and Soul Harvest targets? That calls for a reliable and mana efficient single target backup attack. Hence Splinters - 210% damage (from 3 darts) for only 24 mana. Very spammable.
Finally, I will note that to the extent that resists and immuinities might be a problem in later difficulties, this build generates Physical, Fire and Poison damage.
Sorry for the rambling explanation, but I'd welcome any thoughts, constructive criticism or improvements anyone has.
Note: A basic assumption of this build is that Cloud of Bats can be held down while you move (holding down a hot key as you move). I assume this since otherwise Cloud of Bats is just a less good redundant version of Hungry Bats (which I am assuming requires you to stand still). If I am wrong, then I would simply go with Hungry Bats (higher damage/auto seeks targets) and the strategy would require running up to mobs and then firing Bats while enemies run at you.
Note 2: Another thought is to replace either the Gargantuan or the Dogs with Grasp of the Dead (Either Desperate Grasp or Death is Life for the extra globes (that 5% increase just seems like a DB error to me, I am betting it will be higher in later patches) and to replace Vermin with Grave Injustice. Grave Injustice seems to synergize well with the overall approach of the build and could help keep the GotD almost constantly available, which is helpful for an up close and personal fighting style.
There is footage of Cloud of Bats. It is active in a 10 to 12 yard radius around the Witch Doctor and needs to be held down while the Witch Doctor stands in place - no movement allowed. If movement was allowed, the description would be similar to the description of Strafe ([...] while moving [...]) or Whirlwind ([...] everything in your path [...]). The bigger problem here is that the skill description does not specify the range of the skill in yards.
I have good news about Phantasm, though. Phantasm does 40% weapon damage as physical to all enemies within ten yards of the skill's effect. You are misreading the description as "within 10 yards (of the player). From the description, the skill seems to be able to be targeted at a specific location, not the player. A good indication of this is that the skill has no cooldown and does not drain mana per second. This would mean that Phantasm is like Firebomb's Fire Pit with a larger radius and more damage.
Surround yourself in Phantasm spam, or cast Phantasm in the same place over and over to stack the DoT. Then you can incinerate anything unfortunate enough to stand in the Area of Effect by using Cloud of Bats to add yet another layer of pain. Cast Swallow Your Soul when you're at 25% or 50% mana to keep the battle going.
I think the skills you are looking for are Rain of Corpses and Manitou, which both seem to offer close-range AoE ability as well as mobility. Summon Zombie Dogs' Final Gift and Grasp of the Dead's Death is Life can also both help you get more Globes.
My edits to the build can be found here.
Cloud of Bats synchronizes better with Manitou, as you can have both Cloud and Manitou up at the same time more easily than Phantasm - Phantasm is riskier and has more room for error.
Cast Manitou first, then cast Death is Life at your feet right as the enemies come into range, then cast Swallow Your Soul followed by Cloud of Bats. Doing this should yield a sizable attack buff from Soul Harvest and health globes.
Gargantuan seemed fairly useless, so I scrapped him and replaced him with Sacrifice's Provoke the Pack, which adds yet another damage buff and enables you to Sacrifice your Final Gift dogs to yield Health Globes.
Survivability shouldn't be a problem as you will be continually healed by health globes, and Swallow Your Soul will provide you with mana if managed effectively.
Hmm, that is disappointing news about Cloud of Bats. I suppose only real experience with the final game will reveal whether Cloud or Hungry Bats is more appropriate (since it looks like I'll have to stand still either way). However, I hope you are right about Phantasm, since that makes the skill even more handy for stacking DoT. I agree with you completely that Grasp might be of more overall use than Gargantuan (if I am going to keep one of the summons, it should be the one that provides more distraction targets). The more I use Grasp in the Beta the more I love it, even without runes. I still hope that the Death is Life effect ends up being more than the paltry 5% boost that is currently listed.
Overall it sounds like I might have to modify the approach to a "stop in one place, create a DoT killzone in front of me with Phatasm/Grasp and just torch away with whichever version of Firebats ends up being the most effective in practice - collect the tasty globes from the ashes - rinse - repeat. And of course using Sould Harvest every time it comes off cooldown. One of the design philosophies here is to not "waste" any more of my precious 6 skill slots than I need to with skills that will only get used occasionally. I would prefer to be able to constantly deploy 3 medium damage skills stacked on top of each other than to have 3 high damage skills that could only be used one at a time.
As for Sacrafice, as a huge fan of Necros using Corpse Explosion from D2, I want so badly for this skill to be awesome. However, it is hard to tell from just reading the numbers/description if it will be worth the slot. The main concern is the long cooldown on summoning new Dogs PLUS the fact that the datamined info about Sacraficce seems to imply that you can only blow up the dogs you have summoned (ie. no chance of optimizing a build with all the chance-to-summon-a-dog skills and passives). But I am open to seeing how it plays out in practice once that game is out.
Dead Rush is now a big nova-like nuke, but with a hefty cooldown, which can be reduced nicely with Grave Injustice. Even then it's not usable all the time, so Angry Chicken comes to rescue as the next best thing. It's essentially half a Dead Rush with a fitting cooldown; these can be used two-three times between each.
Grasp of the Dead has become a crowd control giant with the buffed slow, reducing enemy movement to a by yet another 50% to a mere fifth seems like the best rune option to me.
Now, since both of the previously-great rune effects of Sacrifice have been nerfed bad, and all the rune effects of Zombie Dogs are almost comically inconsequential, I find it better to get my meat shields from a skill that does something else as well. Mass Confusion and the Devolution rune are as good as it gets here; Now I can benefit from the cooldown reduction even if the dogs wouldn't need a recast.
Soul Harvest is a good addition to almost any build. Swallow Your Soul is the rune choice as long as I'm using Pirece the Veil passive. I will probably have to change it to something more defensive, such as Jungle Fortitude, and then I'm not too sure which rune would do best, but maybe the healing one.
My choice for the main spammable damage skill is Explosive Toads, since I think it suits the build's idea to stay close to the enemies, but doesn't hinder the movement of the build in such a way as Cloud of Bats, for example, does. Addling Toads would be another good rune option for more safety, or Toad Affinity for better mana efficiency.
Some notes about your mana cost concerns: Baseline spell regen is 25 mana/sec, and it is constant throughout the leveling process. Mana costs, likewise, stay the same despite leveling. In the case of fire bats, the damage ticks and mana costs occur at a rate equivalent to your spell cast rate (attack speed). As such, at 27 mana, fire bats has similar efficiency to Poison Dart. Since Cloud of Bats can only be cast while stationary, you could get rid of Splinters for Hungry bats at 200% fire damage and have a combo aoe/st spell.
To replace the AOE loss of Cloud of Bats, you may want to consider Plague of Toads with Toad Affinity. Affinity would provide you with limitless AOE damage, and allow you to swap Swallow Your Soul for Siphon on Soul Harvest. I'm personally not a huge fan of the Toads, because they work best in close quarters, but that's what you want anyway, so it works out nicely.