You don't really need a lot of hit.
It is more useful then anything else, until you hit the cap for your content.

For heroics, with the hit talents in Balance.
You need 52 or 80 rating.
Forget which was which, but Shaman's needed one and Druids the other.
Regardless, a very low amount of hit rating, probably 2-3 pieces or so.

Spell power is more important then anything else.
But if everything is equal, crit is more useful for boomkins then haste or spell penetration.
If you're PvPing, put the spell pen. enchant on the cloak, and that's probably enough.