One really small question. I have read in several papers that rendering clouds is really really difficult. Now I want to write down all the reasons WHY this task is so hard. Currently only one point came into my mind. + their complex ever changing shape Which additional points make cloud rendering so difficult?

With most solid, opaque materials, the color of light reflected from any given point only depends on properties at the surface. Clouds exhibit subsurface scattering, which means the light can be scattered many times inside the cloud before being reflected back out. Since simulating this process exactly is computationally infeasible, you have to find a method to approximate how much light is scattered in any given volume. The same problem applies to a lot of common materials like skin, wax and marble.

The main reason rendering clouds is so hard to do well is that rendering soft volumes is not what a polygon rasterizer is best suited for...so lots of tricks and hacks have to be used to get something reasonable.