Speaking from my experience of using Groovy as part of a large Java
J2EE project, there were several factors that made Groovy stand out.
The component being targeted by a language other than Java involved
alot of complicated math and data set manipulation. Like Ruby, Groovy has really nice language support for manipulating collections of object. Unlike many of the other languages including Ruby,Python and Rexx, Groovy naturally understands Java classes and can be precompiled into Java byte code so deployment is a piece of cake. To the corporate infrastructure folks, it's just another Java jar.
One area that Groovy stood out from ALL of the other languages we looked at (besides Rexx) was the native BigDecimal support. Since accuracy is important in any business application that deals with money, this was a big plus. Using a float or double would not be appropriate for any of the decimal attributes we use. All of the other languages we looked required explicit calls to create BigDecimal objects.
Groovy has many other great features but these were the ones that were significant given the business domain and the need to integrate into a Java environment.