The first work to be done is to make a choice on the right framework or library that allows us to connect node.js to a database (which can be MySQL or can be something that fits better to node.js) and then a library for having a user management using node.js and that database and which could use the bitcoin address as username. If nothing like this already exists it would be the first piece to be developed.
How did you reach the design choice "node.js"? Why not rails, django, play or anything different? What do you see in node? I'm always interested in working on cool projects that bring new things to the world but I wouldn't want to without knowing anything. The basics you described can be easily implemented in any programming language however what is the goal?
The main reason is because node.js is non-blocking and I had very good experience with node.js applications. Others may be non-blocking too. But a node.js http server is very lightweight too. It only takes a few lines to create a basic server.
I think everything could be made in every language.
But as node.js is JavaScript it can be programmed more easily than having to know ruby, python or other languages.
I have thought about doing it in Java (play would be a useable framework), but then I decided it would be better to not have a Java VM running which always takes some more ressources than others which would not be the problem with a good root server. Basing on JavaScript instead of Ruby, Java, Python or PHP (most people know PHP but it becomes insecure very fast when you are not an expert) allows more people to help programming, I guess.