My two take aways are, generally you can make anything scale, cache like hell, and I personally don't see enough value in .NET to justify the licensing costs to roll it out initially or the long term.
One point on the cost issue. To an individual a Windows server license looks like a lot of money but to a business it really doesn't matter. Every server my company buys has many paying customers tied to it so it really doesn't matter to us at all.
Besides the monetary cost, there's the opportunity cost of dealing with licenses in the first place. Part is compliance (Does your company have current licenses that cover every bit of software on every virtual machine on every developer laptop? Can you prove it?) and part is procurement (Do you get the plan with free upgrades, or do you buy new? Will you need enough licenses over the next two years that you should get a site-license or is it cheaper to stick with single user licenses?).
I always thought that, at scale, SO and (Facebook or Twitter) were an apple to oranges comparison. Not because of load, but because of types of load.
For whatever reason, I have it in my head that the difficulty Facebook and Twitter (and even Digg) face in scaling are the social aspects of their sites. These are the things that require custom software (FlockDB and Cassandra) and a lot of machines.
Perhaps I need to use SO again, but in the day, this social aspect of SO didn't exist. This means their scaling challenges are far more traditional, say like slashdot. 99% cacheable reads type thing.
If I'm right, SO is really just a case study that, depending on what they are doing, some startups will be able to scale with .NET.
