The only two things that'll make me switch out of my current MBA 13":
8GB RAM + HD 4000 iGPU.
The iGPU is a given, the RAM is not. I'm really hoping for that 4GB bump though.
Storage space isn't a big deal. I've been using a 256GB SSD for a couple years now with an external drive to store my music (moved to iCloud now), photos (50GB of travel photos aren't required day to day), and ISO images (MSDN member, saves me from downloading them again).
I use about half of my 256GB SSD for day to day work.
Yeah, I agree, 256 is the sweet spot right now for me as well. I have 128gb on my 11" and I'm upgrading it to 256. The laptop is just my travel laptop, but with 128gb I find myself constrained to fit all the movies (in the HD quality) that I want to fit to watch on planes\hotels. 256 is an absolute necessary for me, and I just plugin to my iMac's 2TB when at home