Lack of SD card = simply unacceptable.
I admit, I much prefer to have ports built into my device, not having to carry external accessories, especially at Apple's price points.
Extra battery is key as well. Some may not care - I do. In fact, battery life is the first thing I check for in any mobile device I buy these days.
Every machine I have must be a powerhouse. Period. No compromises. The 13" Air pales in comparison to my i7 17", but that's okay - everything pales in comparison to "The Big Apple". The Air manages to run VMs fine (SSD), and deal with most basic applications such as Office and whatnot, and while the battery does take somewhat of a hit, it does get the job done at the core.
And yes, I've owned the 11" Air and yes, I did like its form factor. But it's too much compromise.
Very few people will have any problem running their stuff on a MBA with 4G of RAM. When its swapping to SSD, you do not have the same slow down that you get with a traditional hard drive.
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You can't judge RAM needs the same way when you are using an SSD.
The goal is to prevent paging. Even if it's an SSD, RAM is still going to provide superior performance to paging. RAM is connected through a dedicated slot, not shared with your file access, OS access, etc. It's one of the reasons I still prefer at least the current Pro over the Air for day-to-day usage - it's got 8GB of RAM and I could go to 16GB if I were a really heavy user.
The other thing to consider is that on the Pro, should the drive die, I can replace it myself. With the Air, you're returning the whole unit. Yes, there's AppleCare, but think of how long you'll be without your machine. Versus the Pro where if my Samsung SSD dies, I can drive right down to Fry's and get another one and be up and running again in two hours. That time savings and retained productivity is well worth my preference for the Pro. Some people aren't thinking of every factor when they're buying the Air.