Even seeing Meteor from a prototyping perspective it looks like a great tool. The integration with MongoDB (3.0 is fantastic by the way) and ability to quickly create and work with collections from within your Meteor applications without really needing to glue anything together or configure is something that appeals to me (especially as a front-end developer who is asked to prototype things quite a lot). Coupled with the support for authentication, user accounts and easy deployment, these are definitely key features to a modern web application.
Windows support is definitely an added bonus, especially for me who switches between a MacBook Pro and a PC for .NET development.