Ahh, sleep. Slumber. God's gift to mortals.
Sleep is all about comfort. As many a mattress commercial has drilled into my head, not all mattresses are comfortable. There is a supposedly optimal point of squishiness and firmness and pillowness and sleepnumberness and weird-yoga-guy-meditating-on-a-mattress-for-no-apparent-reasonness. In short, different people like different mattresses.
The generic want for a mattress, however, is relatively universal. This is due to the way gravity acts on the human body. Since we are not simply a point mass, gravity pulls on our entire body. When laying down, as if to sleep, this pulls us towards the ground. Due to the natural contours of the human body, the actual contact patch with a flat floor is relatively small, leading to high pressure areas (since pressure is inversely related to area, with a smaller area a greater pressure is produced). It is this pressure that is considered uncomfortable.
A good mattress will conform to one's body, greatly increasing the size of the contact patch between human and bed. Speaking of beds and mattresses, hot damn one would be nice right about now...