So now we know when the expansion is coming (Sept. 25), and we also know when patch 5.0.4 is coming (Aug. 28). That means we'll be living with this patch and all of its sweeping changes to the systems and mechanics we're currently used to for the better part of a month before we get a whole bunch of new content to distract us from how different everything just became. That's a substantial amount of time to play with the new talent system, new spells, and ... erm ...pet battles that the pre-expansion patch will bring. I think this is a good time to discuss those changes and prepare ourselves mentally for all the shiny new bounty we are about to receive.
New talent system
This is by far the biggest change on the docket. Forget everything you currently know about talents. Gone is the 72-tier tree of talents that until now defined your mage's effectiveness. Gone are cookie-cutter specs that you had conform to or lose damage. Instead, we get six successive choices that are balanced pretty much equally.
Most of the talents we're used to now have either become baseline spells, been rolled together into current spells, or vanished entirely. Every 15 levels, we get to select from three talents. None of these talents are obviously better or worse than the others, freeing us to to choose the one at each tier that suits our playstyle or the needs of a particular situation.