In PHP (or Java/ASP.NET/Ruby) based webservers every client request is instantiated on a new thread. But in Node.js all the clients run on the same thread (they can even share the same variables!) I understand that I/O operations are event based so they don't block the main thread loop.
Note: I've used clustering to get around this, but it's not pretty.
Node.js was created explicitly as an experiment in async processing. The theory was that doing async processing on a single thread could provide more performance and scalability under typical web loads than the typical thread-based implementation.
And you know what? In my opinion that theory's been borne out. A node.js app that isn't doing CPU intensive stuff can run thousands more concurrent connections than Apache or IIS or other thread-based servers.
The single threaded, async nature does make things complicated. But do you honestly think it's more complicated than threading? One race condition can ruin your entire month! Or empty out your thread pool due to some setting somewhere and watch your response time slow to a crawl! Not to mention deadlocks, priority inversions, and all the other gyrations that go with multithreading.
In the end, I don't think it's universally better or worse; it's different, and sometimes it's better and sometimes it's not. Use the right tool for the job.