Feel free to tell me if I'm wrong, but you have to run a windows server to run asp.net and that comes with a whole plethora of maintainance and security issues. There's also the cost factor, you know IIS, MS SQL, etc... If you want the whole M$ package. It gets expensive just to break into it, and even worse on maintainance, as you have to watch out for all those viruses and broken service packs. This is why people say Microsoft Sucks. So there is some technical merit to it.

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.