I have loved and studied clouds for a pretty long time now. I was taking pictures of sunsets from my bedroom's window when I was 15, I'm now 35 and nothing has changed : I'm still amazed by the variety and colors and shapes that cloud bring to an otherwise "sad" blue sky.
My favorites are storm clouds : the humongous cumulo-nimbus.
When it comes to rendering, you have 2 difficulties to overcome : lighting and shape. Both are difficult and very important.
You may underestimate the importance of the shape of a cloud, especially because they look random so you might think a random shape is okay. But then you are plain wrong, because clouds are all but random.
If you watch a time-lapse video of a cloud (e.g. http://www.youtube.com/watch?v=_ROd3yaHw00), you can see how clouds move, how they are created and destroyed. One main observation to make is that accelerated clouds motion make them look like regular smoke. This is an important fractal pattern : clouds are like smoke, but bigger. And as they are orders of magnitude larger, the time scale is orders of magnitude smaller. It will take several minutes for the huge mass of a cloud to accomplish the same motion as smoke. A bit like the time for humanns seems like slow motion as compared to insects.
time lapse => smoke, sinks, sources, etc. fluids