It seems everytime I try to advocate Groovy/Grails to many Java programmers, it just doesn't sell well.
After some thinking, it turned out that the Groovy language is what's putting off many Java programmers. Yes, learning a new language is more difficult than learning a new framework. Probably, if there does exist a framework similar to Grails, but written in Java, that would appeal more.
Unfortunately, many features offered by the Grails framework won't exist without a dynamic language such as Groovy.
IMHO, the real powers of Groovy and Grails stem from taking what was productive from the other platforms. Check some code samples from each of them and come back.
It's so sad that many Java programmers are hurting themselves by believing that the Java language should be used for almost anything related to web development.