The biggest problem with Kotlin is also it's biggest strength: excellent interop with Java. This means that you can use all of the Java libraries...but that also makes writing new libraries in idiomatic Kotlin unnecessary, so you're going to get less of them, and have to use a lot of crummy old Java APIs.
Try Kotlin with the Spring Boot framework. I view Spring Boot as the Rails of the Java world and the Spring team has done a lot of work lately on Kotlin compatibility and idiomatic APIs.
This week I’ve been porting one of my Spring Boot apps to Kotlin and that was a great experience.
The only thing I’m waiting for is good reactive drivers for Postgres and then you can go with their new reactor based apis instead of the older one-thread per request Spring MVC.
This is a good point. I haven't used Spring in forever, so I don't know if/how it takes advantage of any modern Java features, but I could see Java 8/9/10 and (a framework like) Spring being pleasant and productive.
The Spring and Spring Boot teams have done a lot of work on this. IIRC Spring Boot 2.1 is compatible up until Java 11, and Java 12 support should be in 2.2.
Some API's are adapted to take advantage of modern Java features like for example JPA repositories that can return Optional<T> for single row queries and Stream<T> for multi row queries. Some API's have been improved with functional equivalents.
If you combine this with Kotlin you get all the power of the Spring framework together with excellent nullability checks and modern language features that Kotlin brings to the table.
