I believe The biggest differentiator in Play 2 is the reactive programming. Even if you look outside the JVM, NodeJS is the only other web framework providing that level of non-blocking http.
Rapid iteration is probably obvious for PHP, Rails or Django, but on the JVM (or .net), it's pretty rare to see a framework that really supports it well.
Right, but that's what I like about Play. They took Netty, and they took Akka and all the other really nice reactive stuff and integrated it very cleanly.
With Akka you get most of the concurrency benefits you would get from a language like Erlang, but with the benefits of having access to the entire JVM ecosystem.
Rapid iteration is probably obvious for PHP, Rails or Django, but on the JVM (or .net), it's pretty rare to see a framework that really supports it well.