Laptops come in many different sizes and weights. What you intend to do with yours will have a direct bearing on what size you get. If you will be carrying it around a lot, then a light one would suit you best. You'll probably have accessories to carry too, so try the weight out in the shop to be sure it's not all too heavy.
Make sure you get optimum storage and power for the weight you choose; otherwise you'll get frustrated every time you have to use it. In case of a future memory upgrade, find out if you can do it yourself, or if a technician must do it. Battery life is also important if you plan on using your laptop away from a power supply. Some only give you half an hour of battery; others stretch to two hours. You know which you'd rather.
Being portable, the laptop is more likely to suffer from a breakdown, so see you get a decent warranty. You'd want at least a year; three is better but will cost more. And what about networking? If you need to network, then make sure your system is capable of it.
