I do notice that a lot of people have other machines in addition to their 11 in. MBA. I wonder if somebody who uses the 11 as their primary machine can chime in?
I use the 11" as my primary. Previous unit was a 13" "ultra-portable" VAIO, which is still a beauty, but feels like a beast (in a pejorative way) compared to my new Air. Aside from a horrible AppleCare experience arising from purchasing a DOA unit, I have zero qualms the 11" other than that Left4Dead2 gets laggy sometimes, which is crucial when I'm trying to fight hordes of zombies.
CPU speed (mine is 1.6, with 4GB ram since both are non-upgradable) is great, and I rarely lag since I don't use very demanding programs like Photoshop or Aperture. And I've seen that a base Air can even play COD, so you really don't need to worry about being best friends with the loading colour wheel.
Battery life is excellent...I've gone an entire day of 8-9 hours on a single charge, and it more than handles my six straight hours of class when I only use MS Word and browse the internet, although I push the limit when I start sneakily watching YouTube clips.
Screen resolution is fantastic and I have never had eye strain; you won't notice how small the screen is after a while. I'd say that it's too good...I'm noticing how poor the quality is of my 700MB downloaded movies now, and I think I'll have to re-download in HD. And I personally don't mind watching stuff on an 11" screen when I'm not at home.
SD card slot? Are you really going to use it on a constant basis? I have a nine-in-one port that can read any card, and plugs into my USB port.
Backlit keyboard is always a beloved feature, but I find that the only times I'm really aching for it is when I'm simply too damn lazy to get up from my bed or chair and turn on the light.
I can't really think of anything else that a student would require out of a MacBook Air...if you're still having trouble deciding, just think of this mantra: Go big or go home. Or, in the case of the MacBook Air, go air or go home. Get the 11.6".
PS. One bonus feature is that you can quietly laugh to yourself when your friends complain about back pain from lugging around a 4-6lbs notebook and textbooks. The 11" and 13" only have a difference of 0.6lbs, but it still counts!