The FIRST thing you need to worry about is your hit cap. 289 is your "magic" number, provided you have Precision and Arcane Focus and you are Arcane speced. Arcane Focus is only for arcane spells, despite its slightly confusing wording.
Picking up both of these will require a 6 point skill investment. Draenai are the only ones lucky enough to get a bonus with Heroic Presence, lowering their number to 263. This also assumes the mage is Arcane speced. These numbers will ensure that the majority of your casts will hit. If you are a Fire mage (not quite as powerful in a raid situation, but still a decent damage dealer), your numbers are 368 and 342 for Draenai
After that, I recommend working on both your Spellpower and your Crit. Arcane and fire both have nice crit bonuses built in if you spec correctly, but having more Crit is never a bad thing. Fully raid buffed, my mage was just shy of 40% crit. I was basically was critting almost every other hit. (This was in Ulduar and fully Naxx geared, btw.)
After that, I'd worry about Haste, then your MP5. And yes, MP5 is of slight importance to a mage. This is why Blizz has made Spirit more useful.
