C# - tied to MS stack. I know, there's Mono, but I'm somehow sceptical using it for a web app...
Python, Ruby, PHP - slow, much worse IDE support than Java.
Clojure - dynamic typing -> worse IDE support, less readable than Java.
Scala - slow compilation, overly complex and unelegant syntax.
Go - looks good, but sometimes feels too low-level - it's a systems programming language, it was created as a C++ replacement.
Dart - unfortunately the ecosystem isn't evolved yet, but this is how Java should look like. The only thing I don't like is optional typing.