I'm a long-time DirectX developer and a recent functional programming nutcase, and I was already excited to try F# before seeing this. Seeing this interactive demo knocked my socks off. There is amazing potential behind interactive simulations like this--experimenting
with various renderers, techniques and even engines interactively can immensely speed up development and tweaking--you always have to tweak settings to make things look right. With this interactivity, you no longer have to build a special GUI for tweakability.
And physics! Tweaking physics parameters on the fly and observing their effects. Ow, what a bucket of PURE joy.