Memory: you will most likely end up having to run virtual machines at some point, or else dual boot. It's nice to have plenty of test environments without having to restart all the time. 6-8gb will do.
Hard disk: to hold all the software and virtual machines you may end up using.
Screen resolution: Makes a world of a difference. I have a 1050p screen on my MBP and it's the main reason why I got it.
Everything else is secondary, but a reasonably sized laptop with 1600x900 or higher will likely be over $1000. --- El Psy Congroo