You shouldn't have to worry about damage type. There MAY be some monsters with resistances, but hopefully they have little effect, and there's no need to worry about immunities.
You're not alone my friend. I too am curious about how skills will actually fare in practice, but I can't help but obsess until then. I don't really have much else to say about your spec. I doubt vision quest is potent enough to support spam of any skill that is listed as over 100 mana, even with effects like haunt's mana return. This isn't a problem as you can run away in between casts or something, but you may find that having haunt and spirit barrage doesn't benefit you because they are both high mana cost and they're both sortof single target. Corpse spiders and poison dart are incredibly cheap options to consider.
I am starting WD and will probably have vision quest set the majority of the time. If you care to see, this is approx what I'm going to use when it's all unlocked, subject to opinions I form after having a chance to use everything:http://us.battle.net/d3/en/calculator/witch-doctor#fkWdYT!UWX!cYZZcc
Summons are nice for vision quest and don't cost much mana (you generally summon them in between fights when you're full anyway). Soul harvest is cheap, heals if stuff is hitting me melee, and boosts damage considerably, so worth keeping on cooldown/active. Toad of hugeness is just so cool looking and seems powerful. Hopefully it can damage bosses, though I don't expect it to CC them at all. Also works with vision quest, but very short cooldown. Pile on is great damage per mana and perfect for vision quest. Zombie charger as my spam attack. I will be prioritizing toad of hugeness and pile on over this and probably running from stuff constantly in between casts or when out of mana. If I had one more skill option, I'd probably take spirit barrage with one of the effects that summons a spectre to attack for a while.
I don't have any sort of escape, so I'm reliant on pets, healing, and killing fast. Zombie charger is the skill most likely to be replaced depending on what I like, but I'm probably going to use something in that slot for aoe and maybe swap in single target but only for certain bosses. If toad of hugeness doesn't work out well, I'd probably use either corpse spiders or poison dart as a cheap spam/single target spell. I might use spiritual attunement, but for now I think blood ritual is more powerful. The hp regen between fights will be nice, but the mana cost reduction is probably less effective then the boost in mana regen. It all depends on how much max mana we end up with. 15% reduced mana costs vs 4% max mana regen per second. A 30-50 mana per second reduction would be even with about 625-1042 max mana, and I think we'll have much more than that at 60.