Weight, thinness AND footprint are important to me, but not actual deal killers for me. The decision to go with a MBAir is a bit more complex than that.
I like my laptops lighter not because I loathe carrying 5lb vs 3lb. The strap on my Incase bag will break if I attempt to shove more things into my bag. Now, you can argue whether or not I really need an iPhone AV kit and a laptop AV kit, a ton of cables, random knickknacks, and still slip a pen and a notepad into the case, but if my laptop weighed less that means a few extra things. My bag is heavy enough now, with a MacBook, that it will trigger the passenger seatbelt warning should I place it next to me in my car. No, I'm not a packrat, really, I'm not... I just hate going around in a day depending on other people to provide x/y/z for a presentation, or lack something at a customer's site, or...
Thinness ends up being a derivation of the above. The bag has finite volume. It's possibly my fault for using an Incase Canvas Vertical Sling instead of something larger, but I also don't want the associated bulk of a larger bag. It then would not fit under the seat in front of me on an airplane, and...
Footprint is actually a toss-up. I realize, even at 26, my eyes are going on me. Genetics and a lifetime of staring at various computers for 18 hours a day I suppose. A few years back, my company got me a 12" PowerBook G4 since I did enough traveling. I started using my personal MacBook wherever possible, not only because it was faster and ran WoW better after work, but also because the screen was larger. However, that very same 12" PB did teach me that it's a bit more comfortable in economy with a smaller laptop. I've seen people take 17", even 20" laptops... luggables? into economy. The 20" ones spill over several trays. It's hilarious, and uncomfortable looking... Anyway, the upshot on footprint for me is that I'll accept the MBAir's compromise, because I can at least use the thing. I can already use a MacBook in economy (no tray table, just on my lap), and so being the same is not a bad thing in this case...
I'll also take the opportunity to shed some of the crud in my bag with the self-imposed "new paradigm" coming in with the Air (read: excuse to toss stuff I can't use with a MBAir) and my MacBook's inverter was going anyway (this is an excuse -- getting it fixed is WAY cheaper than an Air).
I gave up on compromised laptop decisions. I don't want to have another MacBook again because it's not what I had in mind. It's not powerful enough for its weight/size class -- it's almost as big/heavy as a 15" MBP. I don't want a MBP to replace the MB because it's really a bit too big for cramped spaces. I'll stop short of saying the MBAir was just right, but at least it gets closer to my ideal travel companion. I'll get something else down the line, perhaps a used MBP, to serve as my high performance mobile machine.