Library support mainly. Node.js is awesome, but the ecosystem is still very very early. Libraries are almost always alpha quality and don't have the level of polish you'll find in Ruby libraries that have been around a couple years. If you need something specific in Node, 9 times out of 10, you have to implement it yourself. I imagine in a couple years, it will catch up with Ruby.

For example: the MongoDB libraries for Node gave us many problems. I won't go into the specifics as that would take a while, but switching to Ruby mongo driver along with the Mongoid ORM resulted in a huge increase in code readability and is much more reliable I think.

Also being able to use amazing Ruby libraries like devise/omniauth are a big win I think. Similar with Jammit, Barista, Rspec, the list goes on. Here's our current Gemfile: https://gist.github.com/68f0326a9480266d462e. Some of these libraries have Node.js equivalents, but they are all very early stage.

That said, most of our code is UI side anyways. For that I'm using coffeescript / jquery / backbone.js. We'll probably reintroduce some small, focused Node.js server components in the future for the most highly accessed http requests as we start to ramp up the traffic.