Kotlin interfaces are essentially the same as Java interfaces. But Traits in Scala are more like (slightly) restricted multiple inheritance. They can have fields, they can be stacked (http://www.artima.com/scalazine/articles/stackable_trait_pat...), you can restrict the types where a trait can be mixed in.
Value types are useful to enforce certain types which helps to avoid million dollar mistakes like the NASA made when mixing inch and meter as simple floating point numbers.
All in all I think, that Kotlin is a good example of the 80:20 rule. You get the most useful concepts and convenience with a low investment. Maybe Kotlin hits a sweet spot in the JVM landscape.
"Kotlin interfaces are essentially the same as Java interfaces."
Interfaces in Kotlin offer functionality beyond interfaces in Java. In Java interfaces can only have public methods. In Kotlin interfaces can also have (abstract) variables. Both, methods and vars, in Kotlin interfaces can be public and also protected. You need this if you don't want to break the encapsulation of your classes that implement an interface.
Value types are useful to enforce certain types which helps to avoid million dollar mistakes like the NASA made when mixing inch and meter as simple floating point numbers.
All in all I think, that Kotlin is a good example of the 80:20 rule. You get the most useful concepts and convenience with a low investment. Maybe Kotlin hits a sweet spot in the JVM landscape.