I've started to move away from .NET and into Node.js. There are several reasons:
Remove my dependency on Windows
I bought a Mac in late 2011 so I could create iOS apps. But besides XCode, my other development tools (Visual Studio, SQL Server) were Windows based so I had to fire up Parallels (a virtual machine that will run Windows and Linux on a Mac) in order to code.
That's the only time I turn to Windows. It seemed silly to resort to Windows to code when there are other tools out there that work just fine, so I wanted to remove that dependency from my daily workflow.
Having just one programming lanaguage to learn and to teach employees will greatly simplify how things are done. The same framworks can be used on client and server, as can the same validation. We've already moved into AngularJS, MongoDB, and JSON web services, so why deconstruct/reconstruct JSON through a .NET middle layer?
Node.js is just plain sexy
Reading up on Node.js, it seems fast. Very, very fast. The asynchronous architecture means that it's very difficult to write blocking code. Everything is asynch, so it's lightning fast.
The reality - it's more fun
That's cool and all, but what's cooler is just doing it in Node. Although I'm sure there are plenty of technical arguments for using another server side framework like .NET or Ruby on Rails, Node is hot right now and I wanted in on the hotness.
Frankly, it's fun to learn and use something new, and I want to have some fun. And if having fun leads to faster apps built quicker, I'm in.