Plus, Bola synergizes with our high mobility and CC - we can afford to wait out the delay. I think the damage/utility of the other 3 needs a buff really, rather than nerfing this.
Decent damge, but single target and costs too much Hatred. Also, forces you to stand still, which is counter-intuitive and doesn't compensate with extra damge or reduced cost. I find Impale for single targets, elemental arrow, or chakram to be far superior. Rather than reduce the cost, I would buff the damage, as you would have to stop/start/stop/start more, causing you to pay that initial 20 hatred just as much.
Fan of Knives
A neat move from Warcraft 3 is very poor in D3. A small AoE that puts the user at high risk and doesn't compensate with high damage. Also, a stupid long cooldown ruins this spell for me. This one really needs work imo.
Rain of vengence
Looks cool, but isn't that effective. Again, I have to wonder if this is for a melee build or something. It suffers from the same issue as Strafe, when would I want to be surrounded to get maximum utility here? Even if you vault/SS in, Shadow Power, and vault/SS out, there's still a huge risk of death and high discipline cost of that combo. I would always rather take Cluster Arrow. Also, a silly 30 second cooldown to top it off and it's not like it clears the screen like those moves in classic beat-em-ups.
In most instances where this is good, elemental arrow is as good, but costs much less. Probably useful only for bosses or elites, when the added CC from frost arrow doesn't work or isn't needed (think Balial). The costs needs to come down to around 15 Hatred, imo.