I'd say a rudimentary knowledge of the Java ecosystem is required, Maven or Gradle are the preferred deps management and build tools. It's a bit of a bummer as quite a bit of 3rd party docs assume you have already written things in Java, and gloss over things like, importing a library. For someone who doesn't have a magic importer (or worse yet, someone who is not using an IDE), you'll literally have to dig around in API docs just to figure out what comes from where (I'm looking RIGHT AT YOU Vertx!!)

Overall, the language feels pretty nice though.

I don't think lack of other JVM knowledge will be a big impediment, but it might be a small annoyance bootstrapping a project (e.g. how to do dep mgmt) and debugging (how to read a stack trace).

jetbrains is doing a very good job of creating good error messages, but just like any other jvm language, you can run into huge, messy, jvm stacktraces. In my experience, Java/jvm proficiency is useful