This is an interesting bit from Celestalon about the coming item level squish:
@Deektion We are. 1-60 was linear, 60-90 exponential. We're extending that linear climb higher (still deciding where, likely to 85).- Celestalon (@Celestalon) December 11, 2013
Right now item levels travel upwards from 1-60 in a linear fashion; each level gained for the player allows for essentially one item level gain. Once you hit expansion content, the item levels jump exponentially, which is why you feel so much more powerful in new expansion gear compared to old gear. This might seem a bit math-y to some, but it's an important point.
As the exponential curve is moved to later in the game's levels, we'll see a more linear progression of gear up front. This means that while you'll get some gear bump when leveling an alt and moving it to an expansion's content, you won't get as big of a boost as you do right now. This isn't great if you wanted to solo older content with your level 70 mage -- but it has no baring on your ability to solo old content with your level 100 mage.