Scripting in Java
I'm responsible for a fairly large hybrid Java/Jython system.   We use java for core API development, then wire Java objects together using Jython.   This is in a scientific computing environment where we need to be able to put together ad-hoc data analysis scripts quickly.
If I were starting this system from scratch today, I would not choose Jython as the scripting language.   I like Python fine, but I frequently encounter awkward mismatches between the Python type system and the Java type system.   For example, if you just want a hashtable, should you use a Python dictionary or a Java HashMap?   The decision might be different depending on whether you are just using the data structure locally in Python code or passing it across the Java boundary.   Jython does a certain amount of type coercion for you, but it's not perfect.  It's annoying to even have to think about issues like this when the purpose of using a scripting language in the first place is to enhance your productivity. 
I assume JavaScript or JRuby would have similar issues.  Today I would choose a scripting language that is specifically targeted to the JVM and leverages the Java type system.   The obvious candidates are Groovy and Beanshell; Groovy seems to have been picking up momentum lately so I'd look most closely at it.
Me and some friends are writing a MORPG in Java, and we would like to use a scripting language to, eg. to create quests.
We have non experience with scripting in Java. We have used Python, but we are very inexperienced with it. One of us also have used Javascript. 
What scripting language should we use?
What scripting language should we not use?