As someone who writes Scala most days, this feature of Kotlin looks pretty great.
No checked exceptions, which for me is a dealbreaker.
I sympathize, I think checked exceptions are a good idea overall, just poorly used in Java, but the current trend of languages is clearly showing that we're moving away from the idea. None of the languages that came out on the JVM these past ten years (Scala, Groovy, Clojure, Kotlin, Ceylon, etc...) support checked exceptions.
No checked exceptions is a deal breaker for you for a programming language? What kind of deals are you making?