From my point of view, the most important feature of the coming Java 8 language is functional programming. In other words, closures, method references, default methods and the improved collections API that enables you to use these features. You can see a number of code samples here.
Of course, languages like Scala and Groovy know these features since ages. So it's an obvious idea to explain functional programming in Java 8 by comparing Scala and Java 8. This blog provides you with a lot of explanations and code snippets in both languages.