1. Someone told me that the KBC clear ones were PMMA (acrylic). I also read somewhere in the wiki that the Zowie Celeritas has Nylon ones. You can get aftermarket Metal (aluminium and titanium) keycas on Taobao (ebay for china)
2. It has to do with the molud, but also seems to be related to the plastic used. The thickness of the keycap can also have an effect, and will change the bottom-out sound slightly.
For example, my ABS keys from WASD are pretty smooth. I don't like them nearly as much as the righer PBT keys from unicomp. The PBT keys I got from Signature plastics are also a little rougher, and feel much better to my fingers. The ABS doubleshot keys I got on my WYSE are somewhat less smooth, but they don't feel rough, more like a river-smooth stone, or a pebbly creekbed shrunk down.
I still think it has more to do with the shape (for example the top of the SP PBT key seems a little smaller than the others, which I like. It also has a sharper angle at the top, which is pretty cool.
I'm gonna try to order DVORAK PBT alphanumeric keys sometime soon, so I'll be able to compare them.