I find one of the most important things is screen space. And I don't mean a gigantic 17" screen; I mean DPI. I have a 15.4" laptop with a 1680x1050 screen; this gives me room to work. I've seen laptops the same size with 1280x800 screens; I don't know how anyone could do much of anything on those.
Don't get anything too heavy. Mine's just over 6 pounds, and that's already pushing it.
Finally, don't worry about hard disk space; if you need lots, just grab an external drive; they're cheap these days.
I use a 15.4" Dell Latitude D830 @ 1920x1200 and wouldn't trade it for any other laptop made. I'd say 1920x1200 is about my limit for a 15" LCD, but if anyone makes a 17" laptop that goes higher I'll be first in line. It's amazed me for years how under appreciated the value of high resolution is.
I had one of those screens, and I thought I would love it, but it ended up causing too much eye strain. Major headaches! Now I prefer small screens with moderate DPI on the go, and using a large external LCD when possible.
Right now I'm using a 37" 1080P LCD as my home monitor and an X60 as my laptop. I use the LCD to keep various communication tools open and the large LCD for coding/reading.
I'm using the same laptop + external monitor setup, but I'd actually like to use two external monitors. For me, screen space and RAM are the two important factors.
Don't get anything too heavy. Mine's just over 6 pounds, and that's already pushing it.
Finally, don't worry about hard disk space; if you need lots, just grab an external drive; they're cheap these days.