From this description it seems that Erlang/OTP would be even better choice than Clojure: we literally pack million of concurrect users into 1 high memory AWS instance.
Though Erlang not as modern or elegant as Clojure, but there is also LFE and Joxa.
It depends on how many external libraries you need. We considered both Clojure and Erlang in our project, ended up going with Clojure mostly because of available and easily accessible Java libraries.
I still find it amazing that I can:
1) add a single line to my project.clj (which will set up dependencies and download libs),
2) add a single line to my :import section at the top of the source file,
3) there is no point 3. Just call Java libraries.
But the choice really depends on your application.
Leiningen probably has its root from Maven, which IMHO the killer tool for Java. I've done .NET, a bit of Python, and RoR development/toying/experimentation in the past, and you need Bundler+Rake+RubyGem (at least) to emulate the core (important) feature of Maven.
Noticing that languages built on the Erlang VM seem to be more along the lines of "an X-like on the Erlang VM" instead of "X on the Erlang VM" (like JRuby, IronPython, ClojureScript). Is Erlang's VM less adaptable to existing languages vs. the JVM, etc.?
That's part of it, but it's also just that there's little motivation for using it as a general-purpose VM. Building a precise clone of Python on the Erlang VM would be more difficult than on JVM, but more importantly, it would be pointless — Python does not have the kind of deep-rooted concurrency semantics that are Erlang's raison d'etre. If you try to stick an existing language that doesn't closely resemble Erlang on the Erlang VM, all you're doing is creating a really awkward implementation of that language and not really gaining anything. So you end up with languages that are similar to X but with a huge transfusion of Erlang's semantics so that they're useful.
Though Erlang not as modern or elegant as Clojure, but there is also LFE and Joxa.