Ok. Coming from an Air owner:
1. It feels more sturdy and less flimsy (note: less cheap!) than my $1000+ 15" Dell notebook. Very solid, the Aluminum is nice.
2. Its considerably smaller and much, much lighter than my 15" Dell, but at the same time there is almost NO compromise with Screen Size and Keyboard. The MBA screen is almost as large as my Dell's seemingly, I have not noticed much of a difference going from one to the other. I picked up my Dell the other day after using the MBA for a few days, and honestly, it felt like a gigantic, bulky, brick!
3. Its fast and there are no bugs. No trackpad errors, nothing. I have parallels running with XP and several other apps like PS CS3, Logic 8, and Final Cut Pro 6. All aps run flawlessly with no lag, even when I have Parallels running XP at the same time.
4. Its definitely very, very portable! It makes a regular Macbook look large! Whats nice is that its uber-portable (smaller than a real book or notebook, the paper kind), yet it doesn't compromise and is full size. Apple hit a home run and made it the perfect size, they knew what they were doing.
5. The battery last at least 4 hours, if not very close to 5. Anything saying less is a lie. Then again, who listens to continuous loops of music playing for 2.5 hours while browsing the web with full brightness on? There's a reason we have iPods and screen brightness controls (to conserve battery).
6. It is whisper quiet. Its so quiet, you wonder if its even on sometimes. I've never heard a laptop so quiet. My Dell sounds like a Navy Aircraft next to the MBA. My girlfriend made a comment about this the other day, she was astonished by how quiet it is, and she is definitely not a techie, but she can appreciate good design.
Overall, its an amazing portable. Best Laptop I have ever owned hands-down. Its even better than my Powerbook G4 was at the time that I bought that laptop, and the G4 cost me almost $3000 back in those days. Its an amazing value honestly. The $1700 I paid (student discount) honestly feels cheap for such a great portable. The look, the feel, the performance, everything was executed very well. Nice touches like the backlit keyboard. Feels more expensive than it really is and makes you feel like you got it at a bargain!
Don't listen to the BS out there. The MBA is even better than I expected. Truly the best portable I have ever owned.