I've created several scalable web-based systems using ASP.NET MVC and the .NET framework. There are a lot of rabid anti-Microsoft developers on HN but generally I find the latest Microsoft tools stack up well to the competition. If you're a Microsoft developer still developing in classic ASP or classic ASP.NET and want to develop something scalable you need to move to ASP.NET MVC.

If you're not paying for open source support, the biggest downside really is the licensing costs. Getting a virtual server with Windows Server 2012 / IIS / SQL Server is a lot more expensive than Linux / Apache / MySQL or other open-source based environments. Microsoft attempts to address this with BizSpark which gives you free software license for 3 years for startup companies.