Node.js is an interesting bet for startups. It's hot, yes, and that means that there's a lot of people with a small amount of experience with it. As someone who's worked heavily with Node (and various Node devs) for a while now, it's easy to do very, very wrong. It's not like using Ruby on Rails. The frameworks available for Node.js won't guard you from screwing yourself and your entire team. There are few conventions on how to structure your app. If I tell an experienced Rails dev about a problem I'm having with my Rails app, they can probably navigate to the right directory with their eyes closed. If I tell a Node.js dev I'm having a problem with my Node app, they'll ask for the repo and get back to me in a few minutes.

Without at least one very experienced Node.js dev on a team, the project has an extremely high likelihood of ballooning in complexity by the 3-4 week mark, quickly exceeding the 7-plus-or-minus-2 rule and becoming unmanageable. I've seen it happen multiple times. I've made it happen. It's pretty much unavoidable (unless the devs you have on your team are already senior-level with multiple other technologies, and even that isn't guaranteed to save you).

Node.js is powerful tech - it allows one to build featureful, highly scalable systems and do it damn fast; but it can also break your startup.

Sorry for the tangent, I know this doesn't exactly answer your question. I can't be the only one who's realized this though, so Node.js is probably not a default choice the way Ruby was. If you're looking for "new hotness" points, it seems like Go would be the best bet.