It's fun to use Moore's law to imagine how small and fast computers will be ten years from now, but it's also surprising to think about how far we've come:
Imagine you've got a shiny computer that is identical to a Macbook Air, except that it has the energy efficiency of a machine from 20 years ago. That computer would use so much power that you'd get a mere 2.5 seconds of battery life out of the Air's 50 watt-hour battery instead of the seven hours that the Air actually gets. That is to say, you'd need 10,000 Air batteries to run our hypothetical machine for seven hours. There's no way you'd fit a beast like that into a slim mailing envelope.
This is a combination of Moore's law and the lesser-known "Koomey's law," which says that for the last 60 years, "the electrical efficiency of computation has doubled roughly every year and a half."
It might seem like computers have stalled- clock speeds have hovered between 2 and 3.5 ghz for the last 5+ years- but it's easy to forget that in 2006, 24MB/s off of a 5400rpm hard drive was a fast read speed, 2Gb was a lot of RAM, and a 5-pound, inch-thick laptop that got 4 hours of battery life was about the best we could do in portable computing.*
*I'm increasingly convinced the real reason newer computers feel faster is solid-state drives. Like I've said before, I have put a cheap $60 SSD in everything from a 2006 Dell D620 to a 2008 Lenovo T61p to a 1st-gen MacBook Pro, and they scream past most of the cheap laptops you'd find on display in Best Buy. Clock speeds and RAM only really limit you on high-end graphics applications and gaming.