If we're talking about photo editing...the display is very important, as KMH mentioned.
The thing about most laptops, is that they use a cheap display technology, which isn't ideal for consistent viewing. But also, because it's a laptop, it's only natural that you are going to be using it in different location, looking at it under different lighting conditions and at different viewing angles. All of that will factor against you for consistent photo editing.
If you are going to use an external 'desk top' monitor, then the laptop can still work fairly well. As as mentioned above, you'll likely want a good quality IPS display.
That being said, one of the students in my Lightroom class, has a really nice laptop with an IPS screen and a built-in display calibration system. Pretty nice.