You should compare the MBA against "ultrabooks", IMHO.
Performance wise, the current MBA probably loses.
Design wise, the current MBA can't really be beat.
Maybe the upcoming rMBA 12" fanless might shine through.
This might sound stupid, but some of the new 1080p chromebooks compete with the MBA, especially if ChromeOS is replaced with Linux.
Probably the Dell XPS lineup and the Razer lineup. Both have a similar design philosophy to the rMBPs.
(copy pasting model numbers here)
The DELL XPS XPS15-6842sLV is like a rMBP, except in a Windows body. It's nice, imo, but could be designed better.
The Razer Blade Pro RZ09-01171E11-R3U1 is very nice for a Windows laptop/ultrabook. It still doesn't beat the quality of the rMBP, at least from first glance. The keyboard structure is a bit funky.
My opinion on a good non-mac laptop is the Lenovo Y50 4K (59425943). On price and performance, the Y50 beats the rMBP 13". So far, I haven't found something that beats the 15" rMBP when factoring in price, specs, and design.