I think the soon-to-be-released features anxiety is applicable for any language, it is not unique to Kotlin. A good resource for this is the Kotlin Koans for example or the Kotlin STDlib itself. I also maintain some OSS projects that use these things extensively (pun intended). What's also present in the language is the multiplatform functionality. You can't do this in Java and you probably never will be able to do it. With this I think Kotlin is one of the two languages that's capable of isomorphic behavior all across the board (you can write everything in Kotlin).