Cold Crits in Frost
While previously haste was a higher valued stat for us than crit, as the mage stands right now, I'd still stick with a good balance of haste and crit. There are no magic numbers or caps for either of these stats right now, but the general agreement with a spec like Frost is that 33% or more crit is ideal to get those bit hits on frozen targets. With the Frostburn mastery, they're going to be even bigger hits since it multiplies the base damage of your spell with a percentage from your mastery rating. Even then, none of our procs in frost are based off a crit rating, but rather an application of a chill effect, mostly by Frostbolt. This is still applied to boss mobs, allowing for Deep Freeze and Brain Freeze to proc, even though the actual slow may not be effective. So while you can throw out as critting frostbolts as you like and they'll be helpful to your DPS, the big hitters in our arsenal are connected to frozen targets (to which some things are immune) and procs of Fingers of Frost. FoF has a 30% chance to proc off any chill and BF has a 15% chance. Granted you can get around this a little by rolling your other spells that freeze, like talented Cone of Cold, Frost Nova, and your Water Elemental's Freeze, but two of those spells require you to be at closer range to use effectively, which means more threat issues or a lot of moving, and the other requires a targetted AOE spell that can be very clumsy. Please not that by no means am I NOT telling you to roll through those spells if you're finding yourself short on procs, but limiting yourself to Freeze (and finding a way to make it less awkward) is a good way to have a much smoother rotation. My big point here is that with more haste and more frostbolts, you're likely to get more chill applications anyway, giving you more of those procs frost mages love so much. Until the talent tree changes to say that chill is dependant on crit, I'm not going to go so far as to say forget the Haste and start stacking crit like a holy paladin.
Fire is a different story. With talents like Ignite, you'll find yourself wanting more crit in order to apply more DOTs, which are the basis of Fire's mastery, Flashburn. They're also responsible for your Hot Streak procs and big Pyroblast hits. Beyond that Critical Mass is responsible for a spell crit vulnerability available to everyone in your group or raid to take advantage of (if the stupid warlock doesn't steal your spotlight.) So at first glance, crit is pretty important to Fire. But I don't think I'd discount haste so quickly. Fireball and Frostfire Bolt are both long cast spells and according to the math nerds (ie theory crafters), the chance of an un-Improved Hot Streak proccing are inversely proportional with your crit chance. Wait what? What the heck was Blizzard thinking? Were they trying to give the little guys a heads up on the pyroblast use or something? The difference of course lies in the Improved Hot Streak option. If you find yourself with a high crit chance, it's imperative that you take this talent as it's an assured proc that continues to scale with high crit. That said, haste is still nothing to laugh at. Sure you'll burn through mana a little faster, but take a page from the Arcane book and use your cooldowns to bring it back up to where it should be.
AB Numbers Look Bigger with Crit
The clever title eluded me, ok? Sheesh. Arcane has always been a horse of a different color for mages. It's a non-elemental school that's got a unique mastery talent and an alternating but more defined rotation. The big deal here is your mana is already limited and casting through it faster is not always an ideal way to do things, especially if you're not hitting that hard when you do so. Now of course we have to take into account that AB has it's own damage increasing debuff, but there is a limit of 4 charges on it. Not a single talent in the Arcane tree is based off critical strike chance, although two of them increase your crit as procs. (So you could argue that Focus magic relies on crit, but not your own; that of another person. And the way it's made, pretty much any caster class will give you 100% up time as it is right now.) This is why the previous arcane school of thought was that crit didn't scale with arcane for crap and it should be disregarded as a stat for fire mages. However, that's not entirely true anymore. If you have a higher crit chance, you're going to hit bigger more often, making it very valuable as it has no effect on the mana cost of a spell what-so-ever. If you're just stacking haste, you're going to find yourself at a severe disadvantage here, as you'll run out of mana before you can really use your cooldowns again if you don't manage that regen phase perfectly. I like crit over haste for Arcane, but I wouldn't go so far as to say ignore the haste, either. Haste applies to every single spell you cast, while crit still stands as a chance and is not assured.