For me the limit is its dual-core-ness. I currently use a 13" Air as my main development machine and for
most tasks it's fine. The resolution on the 11" is bearable, although I notice the difference. I also think the screen bezel is ugly at the 11" size.
When it comes to pushing out multiple VMs to simulate various real-life scenarios the 8G limit comes into play and it's not always sufficient. My earlier MBP w/ 16G was fine.
If I could have a 13" Retina Air with 16-32G I'd be a Very Happy Boy. As it is I suspect I'll be moving back to an MBP after the Broadwell shift and I can get 32G DDR4. Until then I likely won't make a change.