Until you reach 17% combined hit from all sources hit is the single most powerful dps increase you can get, point for point. Now if you're deciding between a pair of level 30 gloves with hit rating on them and a level 80 epic the choice is obvious. But for comparable, Hit > everything else.
Keep in mind the 17% includes talents and raid buffs.
As arcane you can have up to 6% hit from talents (3% from arcane talents, 3% from Elemental Precision in Frost).
As Fire or Frost you can only get 3% (again from Elemental Precision). Take note that the popular Fire/Arcane build skips any hit talents in favor of picking up at least 18(?) points in Arcane, precluding you from taking Elemental Precision.
You can gain an additional 3% hit from having a Shadow Priest (Misery, I think) or Moonkin Druid (Improved Faerie Fire) in your raid.
Finally, if you're lame Alliance Draenei have an aura that gives 1% hit to the raid.
Which means if you have all of these things in your favor you have 10% hit from talents and raid buffs alone, before you have any hit on your gear. Finally, the 17% hit only applies for raid bosses which count as 3 levels above you. The amount of hit you need for Heroic bosses and raid trash is significantly lower (there is a large jump in the hit requirement for 2 levels above you and 3 levels above you).
Hopefully this clears it up a little.