Price and performance. Size of the screen is important too. A big screen will be heavier and harder to lug around but too small is hard to view and type on. Do you want a touch screen? Adds a lot of additional cost. Batteries are important but at what cost? The longer the battery lasts the lot more the computer will cost or can you live with being tethered 99% of the time. My battery is kaput - it might last 5 minutes but I can live with being cabled up.
The good thing is price continues to come down and performance is going up. I like brand names but other folks prefer no names.
First thing I would think about is what you are going to use it for. If you're big into gaming and things like that, then you need to go for a higher end computer with good graphics, lots of memory, huge hard drive, big screen, etc. But if you mostly do email and web browsing, maybe some simple photo editing, then you can get away with a much less expensive machine--pretty much anything out there can handle that sort of activity. Also think about whether you'll be traveling with it often or not--if you're a road warrior then having something lightweight will be very beneficial, but if it's going to sit on a desk at home 99% of the time then there's no reason to pay a premium to get something super lightweight.