Better speaker system on the 13" to match the performance of the 15 and 17" models.
Cheaper SSD options. (yeah right, lol)
Lighter overall feel (to be 100% honest, the retina model feels about as heavy as my early 2011 MBP did and it's much thinner, so i was a bit confused when i upgraded).
The cheapER SSD options may be coming... I saw recently that you could get a 960GB SSD for $600. Still pricey, but they are becoming more mainstream, and therefore, more reasonably priced...