I'm getting my MacBook Pro (15" Retina) repaired. In the interim, I'm using a 13" MacBook Air as a replacement.
1) Weight: The Air is so fricking light. I never though of my PRO as particularly heavy but the MBP is just a different league. Turns out a 33% weight reduction is actually a big deal.
2) Screen: Amazed at how noticeable switching to a non-Retina screen is. Everything looks crappy. Shows how used to the Pro's screen I've become without even realizing it.
3) Battery: The Air's battery life is simply amazing. I'm getting over 7 hours on this thing compared to <3 on my Pro. Wonder how much of this is because of the non-Retina screen.
Overall, the Air is highly usable. I do miss the screen real-estate that comes with 15" though.
My ideal computer specs: 14" with Retina screen and form factor of the Air.
Update: The one negative with the Air being so light is that I find myself constantly thinking I've forgotten to bring my computer with me. Good problem to have.