I don't care much about the particular lexicography, whether it's classified as an "ultraportable" or a "subnotebook" or a plain old "laptop." What I want is a (mostly) fully featured portable professional laptop (mini-MBP?) with a 12" or 13" matte LED screen and at least 64 mb of dedicated vram. I don't care if it's a half inch or two inches thick, I want it to weigh less than 4.5 pounds. And I could care less whether it has a backlit keyboard, a blu-ray drive or Firewire 800. And you can go hang a touchscreen, probably be a bitch for those of us who type all day long. I'd go without an iSight camera, too, all in the interests of portability/cost.
I guess the minimum specs for me would be:
full-sized keyboard, 12" or 13" LED screen (I'd probably prefer 13", everything else being equal), at least a 2.0 GHz Intel C2D, 80 GB hard drive (or a 64 GB SSD), 64 mb (or more) of dedicated vram, ideally some sort of CD-RW drive (so you can load programs, etc., though I'd be willing to sacrifice a SuperDrive for size and affordability), at least 2 USB ports, an Ethernet port, an Airport Extreme 802.11n card, all in a stylish, durable package weighing less than 4.5 pounds (ideally, less than 4 lbs.). For that I'd pay $2k -- maybe even more.