Welcome to ~2004 in IBM ThinkPad X-series land, IBM "solved the laptop form factor" ~4 years prior to Apple's Air.
And even before that Sony's PCG-Z505 delivered most of this in a robust and lightweight magnesium package.
No. What goes without saying in my original comment was the fact that the MBA has an aluminum unibody.
Aluminum (or other metal) unibody cases represented a major step forward in aesthetics, substance and durability.
I don't care how interesting the thinkpads are (and they are interesting) - I am not going backwards to plastic.
The X220 I'm typing this on is built like a tank, and I'm quite appreciative the palmrest and keyboard keys at least aren't a thermally conductive metal like the rest of the machine.
Doesn't your MBA use plastic keys as well?
Plus unibody designs mean the whole laptop body needs to be replaced if there is damage instead of just replacing a single panel. High end Dell laptops are almost modular in comparison.
And yet the aviation industry, obsessed with lightness and durability, is eliminating aluminium in favour of plastics with each successive generation.