Node.js ecosystem is lacking a "proper" framework like Rails
Leaving the platform / language issues behind, that's also a false myth. Personally I like Ruby and I like Rails, but I find it a bit overkill for small / medium web projects, and I do like to have ownership over my own code. I'd rather start with a small and manageable codebase and extend it / add modules if I need them, when I need them. That's why I think Express.js offers the ideal balance between lightweightness and extensibility. And Connect, (which Express is built upon) gives you the power or Rack, with an even richer ecosystem, I'd dare say. I know most will compare Express with Sinatra, but I think it's more than that, and again, I've heard even hardcore Rubyists admitting Rails might be a bit too much sometimes...
What about client-side assets management, packaging, fingerprinting?
Surely, there's nothing in the Node.js world comparing with Sprockets? Actually, there is. More than one. Node.js is the place where things like that are born. Bower, Brewer.js, Piler, just to name a few.
EDIT, July 2014:
Things have evolved since I wrote this article. Please consider using ASPAX instead of ASPA.
Also, for more complex scenarios, you might want to have a look at Gulp.
So there's plenty to choose from.