What makes a good razor?

I had this thread in another forum, and was directed here.

I'm curious about the qualities of metal that make the "great shavers" better than the rest.

I know very little about steel, other than differing amounts of carbon make different types of steel, which affect its hardness in some way. I know nothing of tempering, other than it hardens it. How and why are beyond me.

And in the end, I want to know how all these things add up to make the kind of razors that have the highest reputation.