Throughout Cataclysm, the arcane spec has been a strange beast. At the beginning of the expansion, it was solidly the worst mage spec due to the absurdly high cost of Arcane Blast. After some quick patch 4.1 fixes, it became our top-performing spec, especially if we had access to Shard of Woe. From that point on, arcane was a source of potent damage. While fire has become the vogue spec for Dragon Soul, arcane still is a very viable and desirable spec. Going forward into Mists of Pandaria, that all could change ... or the spec could be made even better.
Part of the source of arcane's potency this expansion has been due to how incredibly well the spec scales with intellect. While intellect provides the same spellpower to arcane mages that it does to every other caster, arcane mages' Mana Adept mastery makes the mana increase from intellect critical. Arcane Blast, the pivotal spell in arcane's rotation, has a static mana cost that is only based on mage base mana and not their mana after gear. Because of this, the more intellect you have means the more Arcane Blasts you can fit in at higher mana levels, meaning more damage through mastery.