There are no Windows laptops that rival the air for the price. Especially when it comes to battery life. Also, the trackpad on Apple laptops is above and beyond both the synaptic and sentelic track/touchpads that appear on the majority of Air (or Apple laptop in general) competitors. Fingerworks was a company bought out by Apple to acquire their touch technology and in my opinion it is one of the best acquisitions Apple made.
I have a Gigabyte X11, which when it was launched, was a competitor to the 11" Air. I'm lucky if I get four hours to a charge on the X11 and the sentelic trackpad in it is awful on Windows and it's pretty much useless under Linux. The Air on the other hand, is claimed to get in and around 9 hours for the 11" and more for the 13". Real world examples I've seen people claiming 10 hours. The trackpad on Apple laptops, once you're used to it, all other trackpads pale in comparison. You'll resent having to use a laptop without one. The Synaptics multitouch pad in my Clevo is good, but I still use a mouse because the synaptics is simply nowhere near as good as the Apple trackpad.
The only other thing is the OS. Many people don't like OS X because it does things differently. Both from Windows and Linux. Personally, I like OS X in the regards that it has all the power of Unix and is a good all round desktop experience. Again, it's just a matter of getting used to how the OS works. I also find OS X keyboard shortcuts a lot saner than Windows or Linux default keyboard shortcuts.
