Laptop computing is all about mobility, and battery life is perhaps the most crucial consideration when picking a laptop that's going to be used regularly on the go. If you're in the market for a desktop replacement system -- meaning you'll mostly just leave it on your desk and don't plan on regularly taking it on trips -- battery life isn't quite as critical. Otherwise, pay close attention to how long a laptop's battery will last.
As laptops get slimmer and designers pay more attention to making them sleek and compact, more and more systems use integrated non-removable batteries. The trade off for that sleeker laptop body is that it's impossible to buy a backup battery and swap the two out to double battery life. Finally, always be skeptical of claimed battery life times. The numbers that laptop makers convey often refer to light usage with a dimmed screen. Assume you'll get 1 to 2 hours less than claimed while browsing the Web and running multiple applications -- and possibly even less if you're playing games or doing something else that taxes the computer.
The size and power of a laptop affects how much room there is in the body for a battery (and how quickly it's drained). Next up: finding that size and weight sweet spot.