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Stats weight value is measured in damage per second per point of stats. For example, how much is 1 spell power worth? Knowing these will help you pick the best gear. The weight values are changing constantly but currently you can follow this handy litmus test.
Int > Hit (to cap) > Haste >/= Mastery > Crit
In other words, Intellect on your gear will be your best friend. As you start nearing the level cap, you'll want to keep an eye on your hit rating. As of Cataclysm and patch 4.0, you can simply mouse over your Spell Hit stat on your paper doll and see how you're doing. Once you've made the hit cap, you'll want to stack haste or mastery. Crit isn't nearly as much use to you as the other stats, and should be reforged whenever possible.
In the next section, we will examine the various Shadow Priest stats in great detail.