Hmm, I think we should actually decide this more carefully. I found XML more human-readable than JSON (mostly because I don't like writing curly braces). Have you seen systems with readable config in JSON?

As an example of XML, look at the Hadoop Fair Scheduler config: http://hadoop.apache.org/docs/r1.1.1/fair_scheduler.html#Allocation+File+%28fair-scheduler.xml%29. (There's a sample lower down on that.) I don't think it's that bad despite being XML.