All laptops should allow you to upgrade the hard drive. Almost all will allow RAM upgrades. Some will allow processor upgrades, provided the system can recognize, power, and cool it. It's common to see the CPU soldered to the motherboard, though. Some very high end laptops allow you to change out the video card, but there really aren't many options there.
Also, If there is a version of your laptop that has options which yours doesn't, you can often add those on. For example, my Dell M1210 doesn't have a camera, but I could install one if I bought the bezel from the version with a camera. Same goes for optical drives and other gadgets. Sometimes you can even upgrade to a higher resolution screen.