Splashup.com is .NET, IIS, Windows Server 2003 and MySQL. It's never crashed unless I've borked something and it's been through a Techcrunch, Lifehacker, and Digg on the same day.
The amusing part about the free tools argument is that the cost of tools is such a non-concern in the professional world. I work for large customers on large projects and tools, whether they be Microsoft, Oracle or IBM, no one's worried about that.
Then again, you might be talking about little side projects, my frame of reference is mostly multi-million dollar systems.
Even at large companies, all things being equal, having no licensing costs is a competitive advantage.
Two words: hanging chad
I wouldn't use security as a deciding factor (we're a Rails shop), but if I did, I'd be writing C# now.
I believe asp.net can be run on *nix via mono, and an apache mod. However, my experience with mono is that it sucks (I've never developped in it just use apps developped in it).
Other than advising against the M$ path on an administrative perspective, I can't really give you a fair comparison.