A lot of my friends buy cameras based on the number of megapixels and then expect higher quality images.  Image sensor size (actually sensor density) has a much higher correlation with image quality than the number of megapixels.  Larger sensors capture images with less noise and greater dynamic range than smaller sensors.  Wikipedia has a great graphic that illustrates the size of sensor found in most common cameras (click for original graphic size).  Sites like dpreview.com offer statistics on image density.  Consider sensor density when buying your next camera.

Image sensor format – Wikipedia, the free encyclopedia