More, more, more, and more. There aren't really any "increasing returns" stats in WoW aside from armor penetration, but that's itemized so poorly that stacking it cripples your other stats so much that your damage becomes a joke.
Generally speaking, with purples, focus on ones that itemize AP/crit/hit. Once you're hit capped, focus on AP/crit/haste. With a few notable exceptions (Grotesque Handgrips, I'm looking at you), all the mail armor is pretty well itemized already and anything that's from a higher "level" of content (IE: heroic purples -> naxx10 purples) should be an upgrade regardless.
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