Traits seem like an odd feature to pick. What are you thinking of, precisely? Kotlin interfaces are basically the same: they can contain method and property impls (same as Java 8). Do Scala traits have some additional feature I am unaware of?

I admit to a little bias: I've tried to get into Scala several times, but the bad documentation and often overly complex approach puts me off. A lot of Scala features seem like workarounds for other features sometimes. For instance value types in Scala have so many restrictions that they are barely value types at all (e.g. they can have only one member!) and so their primary use case appears to be implementing overhead free extension methods. But Kotlin supports extension methods directly. So whilst implicits+value types may appear to be two features that Kotlin lacks, when you boil it down, it turns out that it has simpler and faster approaches to the same thing.

That said, real value classes (with JVM support) will be good when they arrive.