It probably differs person to person, but for me, a good UltraPortable is:
1. Light (about 3 pounds) = MB Air
2. Thin = Definitely MB Air
3. No Optical Drive = MB Air, and Apple made it better w/ Remote Disk
4. Still decently powerful = MB Air, 1.8Ghz is enough, and 2GB of RAM is good.
I think Apple nailed the Ultraportable, though if it had a 12 inch screen that would have been nice, but I don't think it's possible without compromising the Keyboard, which they kept the same.
an utlraportable also means mobile = good battery life or swappable batteries, or DUAL batteries. also ultraportables are also still as functional as the regular notebooks
mba is missing the following to be a real sub notebook
cpu should be the same speed, 2-2.4ghz (they went with a crippled cpu because of the thinness, the heatsink is not near big enough for a real cpu
most other 12" have the 2.0GHz cpu
gigabit lan/modem (these usually come together)
3+ usb ports (even the 7" eee pc from asus has 3 usb ports)
firewire, its a mac, it should have it, ALL other laptops have it even if its only the 4 wire connector
external but INCLUDED DVDRW or DVDRAM drive (they give you recovery dvds which is good, but no dvd drive? thanks...)
dual batteries, most acer and asus laptops come with both a small and large battery
mic port? it has a web cam but no mic port?
SPDIF? or optical out? my 4 year old laptop had optical out, my new one has spdif
docking port? it barely has any connectors at all, a docking port would make sense
even the smallest laptops (7"-11.1")have a socket for ram, mini pcie, removeable battery, HDD compartment, why cant the mba have it? btw almost all the other notebooks are cheaper