What a lot of people don't realize is that the narrow gap between the case bottom and the surface it's on is important to cooling. If you put the laptop on a table, the gap is maintained and air can flow between the bottom and the hard surface. When you put it on your lap, or worse, on a sofa or bed, now there is no airflow between the bottom and it won't throw off heat as easily. On your lap it would be better to put it on a board or one of those lap pillows with a hard table top, both of which would preserve the airflow across the bottom of the case.
The worst case scenario would be the laptop on a bed where the blanket or comforter covers not only the bottom, but the large back vent. Then there would be no airflow at all, and nowhere for the accelerating fans to push the hot air.
As far as materials, the current aluminum's pretty good at throwing off heat. The problem is that people don't like aluminum because it gets hot to the touch. But that's because it's moving the heat to the outside, where you can feel it.
