This talk is an awesome explanation of how Groovy is impacted by Java 8. I am specially relieved to hear such a prominent voice such as @venkat_ tell us that Java 8 does not replace Groovy but it makes it stronger.
Moreover, @venkat_ explanation about the differences between Java 8 Lambdas and Groovy Closures is pure gold.
This talk by @kenkousen at SpringOne 2GX could be split in 20 blog posts.
With code examples published at Github he explains topics such as Closure Delegates, Closure Coercion, .each and return statements, Operator Overloading, pros and cons about @TypeCheck, @CompileStatic and @CompileDynamic, transforms such as @Delegate or @Inmutable and many more.
@sflahave writes a cautionary tale about dynamic method invocation in Groovy. I like how the post immerses you into how he found this issue while going through his working day.
In Groovy if you call a single-argument method without passing any arguments, the argument value is filled with null. @glaforge discusses in this post if it may be better to raise an exception instead. Moreover, he illustrates how to achieve this with a closure or method.