I really love Kotlin inline keyword, lambdas without the invokedynamic overhead is very good since they have the exact same performance characteristics of the non-lambda version, and gives you the ability to apply the DRY concept in exchange of binary size (which we [commonly] don't really care in Java anyway, the JVM and Java Library itself are extremely big already).

Kotlin is more functional-ish than Java (we even embrace the immutability idea), maybe High-order functions are underrated by those coming from Java and that are used to imperative thinking way of solving problems.