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 bearing on your ability to solo old content with your level 100 mage.
One thing that does concern me, however, is that simply moving the exponential item level increase later doesn't fix the problem in the long run. What happens in two or three expansions after Warlords? We're going to be right back at Mists numbers, and that's going to necessitate another squish. Given Blizzard's insistence of an increase expansion release schedule, that means we could be looking at a squish every couple years.
While I find this squish necessary and very good for the game, having our numbers constantly decreased at the release of every other expansion or two sets up a situation where you're too often entering an expansion not building forward, but building from behind. I worry about the psychological implications for the mass player base with this; not the ones that understand the numbers (they might not like it, but they'll get it -- the group I suspect most of us reading this fall into, one way or another). I'm worried about the group that just logs in to the expansion and goes "What the hell? I'm weaker again?"
The item squish doesn't seem like it will solve the long term problem, and that's something that'll need to be addressed. I've reached out Celestalon via Twitter for his thoughts, we'll see if he's able to reply. I am more than happy to be proved wrong here -- in fact I hope I am.