If your main reason for buying a laptop is because you don't have enough space on your desk for a desktop, weight may not be as big a factor as it would be if you're buying it because you plan to take it along when you travel. However, the down side of such models could be cramped keyboards and tiny screens. Also, think about how powerful you would need your portable computer to be.
Some models are heavy on the power, but at the same time heavy on the shoulder.
Others are light, yet not powerful at all.