There are plenty of excellent and high ranking sites out there built on various versions of MS/.NET, including a lot of very large sites. Regardless of platform you will want to make sure that you you have one canonical version of your site, and never serve the same content at more than one URL (a common issue with .NET sites).
It's also a very good idea to avoid showing file extensions, so that if you do change platforms later you won't need to deal with lots of redirects.
So in other words this is good:
www.example.com/directory/filename
... and this is bad because it limits future flexibility and doesn't look as appealing to users:
www.example.com/directory/filename.aspx
You might wanna check out our Microsoft IIS Web Server and ASP.NET [webmasterworld.com] forum. ;)
