I'm a real customer with real problems. Grep this page for latchkey's description of it.
The funny thing is, I don't have much sympathy for Rails users. Scaling problems with a single-threaded, serial request-processing architecture? No surprise there. But we have inexplicable H12 problems with Node.js. There's something broken in the system and it isn't random routing.
There's nothing wrong with Node's concurrency. Our app, like most webapps, is I/O bound. Any individual instance should be able to handle thousands of concurrent requests as long as they are all blocked on I/O.
Being able to process more than one concurrent request (as Node can) is "real concurrency". Java-style native threading is a step above and beyond this, and unnecessary for most web applications.