There's a very finite amount of stuff you need to pick up and this is common to JRuby, groovy, scala too. Remember you can ask questions on Stackoverflow, IRC (freenode #clojure, google group, folks are supportive. Re: JVM for non-java programmers:
http://stackoverflow.com/questions/4741626/what-parts-of-the...
http://stackoverflow.com/questions/4230746/does-being-a-comp...
http://marxsoftware.blogspot.com/search/label/Java%20%28Gene...
(Bytecode for dummies, the Hotspot switches post are terrific)
http://stackoverflow.com/questions/5721496/learning-java-so-...
Addendum: My recent pain point was downloading a self-extracting JDK installer from oracle's site, and it wouldn't extract. Also, the hotspot switches are not exactly very finite amounts of time to benchmark, but most people seem to bump the defaults for MaxInlineSize, Xmx, Xss, and Xms
http://www.theserverside.com/discussions/thread.tss?thread_i...
I had some background in sundry Lisps, but my stumbling block was my lack of knowledge about the JVM and things Java. You really do need some experience in the ecosystem, else Clojure discussions about boxing, interfaces, AOT, stack traces, what-have-you will be mystery meat.