Good posts so far. I am really really surprised most of them do not take spec into account.
Spell Power and Hit rating for bosses are always your most important stats, being a bit over hit cap rather than a bit under is recommended. 26.2 hit = 1%, Misery from a shadow priest or imp Fairie Fire from a druid = 3%, heroic presence from a draenei = 1%.
After those two stats, it totally depends on if you are fire/ffb or arcane as to what you want to gear for.
For fireball (18/53/0 etc) and ffb (0/56/15 etc), crit > haste/spi are your next goals. Obviously fire is all about the crit, with how Hot Streak works, and the pros have calculated that crit is as beneficial as haste for dps, even though it takes 45.9 crit compared to 32.8 haste for a 1% increase. This is because fire/ffb has a 245%-320% crit modifier, where as arcane only has a 175% modifier. Spi is very important for fire now because of the passive mana regen added to pyromaniac, as well as molten armor giving more crit per spi. There is a fair bit of spi/crit cloth gear to choose from.
Arcane on the other hand (I like 58/2/11) is ALL about the haste > spi > crit. For arcane, haste is calculated as being twice as much as an improvement for dps than crit, AND it is less expensive than crit when determining item budgets, meaning you can usually get more of it on gear. With 100% mana regen during casting, spi is quite important, although if you have massive raid buffs (sp, shammy, pally) for replenishment you will be able to drop a bit of spi for crit on most encounters. I'm up to almost 700 haste on my boss set and I've been a top dps arcane mage ever since arcane became viable again with 3.08.
I'm not completely sure about frost, since it is not considered a viable raid spec, but I would venture a wager that it is more like arcane where haste is (slightly) more beneficial for dps than crit. I would also guess that for pretty much every boss spi would be near useless compared to haste/crit.
Arcane does do more damage and dps than any other spec atm, IF you have the computer and internet connection to handle it (shorter cast times means lag cuts into your dps more, Quartz can help you overcome this), and if you can wrap your head around the dynamic casting rotations (spell selection is highly dependent on situational buffs, movement and overall boss strategy). If you want a more relaxed approach to mage dps and don't care if you aren't at the top of the meter, fire/ffb still do great damage/dps, and are a bit simpler to adapt to whatever encounter you're doing.