So as an update... Grave injustice + rain of corpses is awesome, two grasps out at once gives a ton of falling corpses. I've found myself casting spells when in spirit form often rather than running, so the frequent spirit walk is pretty fun.
This build as posted, however, NEEDS a very high +pickup radius to work. It needs to be most of the screen. But the build also urgently needs life per second items, as you always get hit eventually and there are no healing spells. Finally, pulling in white mobs for grave injustice tends to result in an empty mana pool when there are enough, so the build probably needs items which help with mana regen. But if your damage is too low, grave injustice becomes pretty useless...
Basically, it is extremely hard to get the right equipment. Grave injustice may work better for a close-up build (dogs, toads etc) but for a kiting DoT build, I've shelved it for now. I have replaced it with Blood Ritual and the switch has made a huge difference.
Really there are a lot of workable WD builds. I've tried a ton of different ones that work well. Problem is... they work on white mobs.
When it comes to elites/champions you realize there's no point in deviating too much from the typical builds, because you need the 4-5 CC spells in order to survive these packs on a regular basis. With the way repair costs are now, it forces you to use these builds even more. When playing alternate builds you can kill white mobs a lot faster, but your chance of dying (losing gold) on elites/champions is a lot higher... to the point where some affixes/monster types feel impossible.
Grave Injustice is really good for killing white mobs, as people know who have used it, but Blizzard has designed a game where elites/champions are wildly stronger than normal mobs which forces you to use the 4-5 CC abilities that everyone else uses plus Spirit Vessel.
The only alternative seems to be getting defensive gear with Zombie Bears + high damage cooldowns. Essentially an offensive build that can still take hits from really strong elites/champions. Since you can afford to drop a lot of damage if you're using Zombie Bears. But this costs way more gold than a dart/bat kiting build and isn't a real option coming into Inferno.
The bottom line is that elites/champions are all that matters unless you have insane gear and can somehow tank them without defensive cooldowns. I don't even think that's possible really, at least not right now.
In D2 you chose the spec that was the most efficient for killing everything because there weren't such extreme discrepancies.
In D3 you automatically choose the spec that can survive crazy elite/champion packs and settle for weaker damage against white mobs.
So, I finished Hell with this DoT build, before the patch today. Didn't start Inferno as I've been busy getting married, but will try it after the patch.
My final build was:
Corpse Spiders - Widowmakers
Grasp - Rain of Corpses
Haunt - Grasping Spirit (30% slow)
Locust Swarm - Flaming Locusts
Spirit Walk - Jaunt
Horrify - the one that gives you a 20% speed boost.
Pierce the Veil, Blood Ritual, Spirit Vessel
Even before the current patch, I had no real problems in Hell despite playing with entirely self found gear. I rarely die (focused on life regen equipment), kill very quickly and move very quickly. I've been using slow 2 handed weapons to get the best out of DoT spells, and with the improvements to 2 handed weapons I am convinced this could be an extremely good Inferno build now.
So, firstly, try this!
Secondly, Widowmaker Spiders are pointless now, so what should I switch them to?
I prefer explosive frogs. The nerfed mana cost makes then usable without spending any mana at all (regen overtakes the cost). They're slow, but they do a ton of damage. I just don't like using Darts because everyone and their mother is all LOL DARTS OR U BAD. =\ Thankfully Blizzard fixed that.
I've gone with Pyrogeist instead of the spiders. I know I can only have one at a time, but with four damaging spells that does not seem to be an issue so far. Act 1 Inferno is easy, up to the skeleton king anyway, will try the next bit tomorrow.