Compared to the others: bad gamut (color reproduction/range). Also, even if it's better than the typical low-resolution 1366x768, it still isn't 1600x1900 or full HD. Brightness- and contrast-wise it's great. But still, there's better displays in the cheaper price range already, like on Zenbook. Or if you want to compare against much cheaper iPad, iPad also has far greater resolution, much better (almost perfect sRGB) gamut, better contrast, and so on. MBAir's display is better than average though, so it's not bad. It's just there's better options available.
Of course, if you're not interested about the color reproduction, this might not affect to you. But if you want to edit photos or see movies like they're supposed to be, then it matters.
I reviewed Thinkpad X1 Carbon, Samsung Series 9 and both Zenbooks recently (tech journo here) at our test lab.
Samsung has the best battery-per-weight ratio, but it has the worst screen out of the three. Still better than average, except its gamut which is typical = bad. The screen is the brightest out of three and 2nd best in contrast after Zenbook. The mouse has some problems, it likes to think you meant to select/drag'n'drop when you just move the cursor and try to click. If you need ultra light, this is the way to go.
Thinkpad's battery life is a bit disappointing, an hour or so less than Samsung's. The screen in general is better, the gamut is almost full sRGB. It's a very solid performer in all aspects. But it's much more expensive; you can get two (cheaper) Zenbooks with the price of one X1 Carbon.
Zenbook (UX31A and UX32VD) has superior display because of the fullHD, almost full sRGB gamut, best contrast ratio and almost as bright display as Samsung. UX31A:s battery lasts IIRC longest out of these, or at least it's similar to Samsung's. UX32VD is a tad cheaper, has the same excellent display, but with power-hungry Nvidia graphics makes its battery last as long as Thinkpad's, so not that long. (3ish hours playing video.) Also, hard drive makes it suck compared to other laptops with SSD.
Out of these, I'd choose UX31A. Good battery life, excellent screen, good performance, light and thin, and not insanely expensive. IMHO not worth the extra 1000 euros/dollars for X1 Carbon.
Oh, and if battery length is the most important thing for a light laptop, MacBook Air 13" is the way to go. Its display is though worst out of these mentioned in this post.
Of course, if you're not interested about the color reproduction, this might not affect to you. But if you want to edit photos or see movies like they're supposed to be, then it matters.