The Air is all about ultra-mobility. When you buy one, youre willfully making sacrifices to get that feature. The ability to travel with your whole media library and all of your apps isnt as important. Being able to run high-performance apps to the very best of their abilities isnt as important. Youre also willing to trade a few hours of battery, an optical drive, Gigabit ethernet and high-speed Firewire.
The MacBook Air, or conventional MacBook? question is actually pretty simple, come to think of it. If youre on the fence, then the answer is get a regular MacBook. If your reaction to the preceding paragraph was Yes, so? then youll be very, very happy with the Air.