I would keep in mind that they have to balance (or have chosen the paradigm of balancing) a skill uniformly across all four difficulties, for all gear levels (casuals and hardcore).
This will lead them to make an ability seem overpowered in normal (as almost all abilities in d2 were in 1-player normal games)...but i doubt it will be that great in higher difficulties in later acts due to proximity to enemies. Besides, 30% isn't that big of a damage boost, and it uses one of your slots (which i believe is shared by the pets?) Gear will make a bigger difference than this skill will.
We can assume that Blizzard is trying to make skills (and classes) balanced, but we shouldn't assume that they've necessarily got it right (yet.)
Soul Harvest does seem fairly ridiculous in normal, and there's a real question as to how much its power level will drop off through the other difficulties. Having to move in close enough to get the buff will be a pretty big balancing factor for the skill in Inferno, but we won't know how risky this really is until we get there. It might be that you can only feasibly use SH regularly if you take defensive passives and use defensive gear. In that case, SH probably won’t be too popular – unless those defensive passives are needed to survive in Inferno anyway.
Having to use up an active skill slot is definitely another balancing factor. At the same time, you can only be casting one spell at a time, which means that active skills that have an ongoing effect beyond the initial cast are valuable.
A 30% damage boost is a big deal. Increased damage means you kill stuff faster, which means you move through levels faster, get loot faster etc. Imagine the reaction of the player base if it was clear that one class regularly did around 30% damage more than the others. Similarly, if Soul Harvest Witch Doctors regularly do around 30% damage more than the others, then there's a problem. There's always going to be some variance between classes and builds, it's not possible to perfectly balance every class and skill. But 30% is absolutely a big deal.
The damage boost you get from gear might dwarf whatever you get from SH, but this doesn't mean SH's damage boost is trivial. We don't have to choose between gear or
SH. We do have to choose between SH and other active skills, which is where the damage comparison becomes more important. If SH ends up consistently providing a 30%+ damage boost (and isn’t too dangerous) then it’s difficult to justify not starting every WD build with SH.
Suppose SH is balanced in Inferno but a bit overpowered in Hell. Depending on how overpowered, this is still potentially a real balance issue. Farming Hell for gold/gear is going to be important, and if WD+SH ends up too far ahead then there’s a problem.
I mean, they essentially gave the monk and barbarian a damage resist bonus because they were dying in inferno difficulty too much, right? This isn't any different.
I'm not sure exactly what point you've trying to make here. Balance only really becomes important in Hell/Inferno and at various points in the leveling game, some classes and skills might seem OP relative to each other, sure. Blizzard makes balance changes that might not seem necessary in the content available on the beta (or even possibly all of normal mode) because they matter in Inferno, sure. But, Soul Harvest hasn't been massively changed, it's just become available earlier. As Uberstein has pointed out, there's been concern for some time about SH's power level, which was never restricted to normal mode/beta levels. The latest discussion has started from people trying SH in beta and going "Gee, this seems pretty powerful right now" but that doesn't mean we don't have to worry about its power later on.
If you're trying to compare to the monk/barb change, then the question is: is SH more like monk/barb now (balanced for Inferno) or monk/barb before their extra damage resist (problematic and in need of change in Inferno)?