"(...)When I compare our performance to similar sites that are running on the open source stack, we are using about 1/10th of the hardware that they are, unfortunately, and maybe that’s because they are not good programmers. But just in terms of the types of queries we are doing and stuff, the Microsoft stack is actually, appears to be paying for itself – in terms of reduced hardware”
Doesn't really provide any real justifications. I have indeed anecdotally heard good things about mssql, but apart from that I have a hard time believing you couldn't get more or less identical performance from jvm/linux/etc (and maybe mysql/postgres really can give mssql a run for its money, but I admittedly know little about relative database performance)
On the other hand, it seems like Jeff/Joel had lots of experience with the MS stack before, so its not an illogical choice. You want to spend your time building a product, not wasting it figuring a new stack you aren't very familiar with.