Kotlin is a promising language, but still pretty new, so it doesn't have as many libraries out there tailored for it yet or questions asked about weird corner cases you might encounter (so it can make it a bit harder to track down bugs). That has its positives if you want to be a pioneer in the language and start popping out some useful open source libraries though.
I love what Jetbrains does though and actively read their blog as they keep improving it. Kotlin has a bright future (I think some of the developers of Groovy are working with Jetbrains on it), but still a language in its infancy.
Scala may be a safer solution because Kotlin is newer, but that doesn't stop Kotlin being a better choice than other older languages like Groovy.
Today the Groovy P.M. announced they're adding statically-typed traits (http://groovy.329449.n5.nabble.com/Adding-Trait-to-Groovy-td...) to Groovy, talking about them like they're an innovation. But they were already implemented by former Groovy++ developer Alex Tkachman (https://code.google.com/p/groovypptest/wiki/Traits) 2 years ago. Groovy added static compilation into Groovy 1 year ago by copying it from Tkachman's Groovy++ codebase, then announced it to the world as "Groovy 2.0" without crediting him. Now they're trying the same trick again, laundering the Groovy++ Traits and passing it all off as "Groovy 2.2".
Alex Tkachman is now one of the Kotlin developers (along with Groovy's creator James Strachan). So which codebase would you trust: one laundered without credit from someone else's implementation (Groovy), or a second try from the actual person who wrote the first attempt (Kotlin)?
Kotlin looks(ed) promising. When I tried Kotlin some time ago they mixed plans, wishes and facts in the "documentation" which made it very hard to do some work, as many "features" in "examples" had not been implemented (And I thought to myself how stupid could I be to not get the examples working)
Yes the official documentation is indeed out of date. I am working to fill the gap (http://goo.gl/TkgqR - in progress)
The potential growth for Kotlin usage will be on Android platform especially after the new Android Studio is based on IntelliJ 13. Kotlin fits nicely here and it's a relatively small language.
Scala though is quite a pioneering language on the JVM. I think a lot of Kotlin features and design are influenced by Scala.
Actually Kotlin has null safety as well[1]. Not sure how generics in Kotlin compare to Ceylon, but it also improved on Java's generic support[2] by eliminating wildcards, type projection, inferred types and some other features.
Random question, but how far along is Ceylon's development compared to Kotlin's? I know they're both relatively new still, but just wondering how feature complete it is so far.
I hate to say it because I like JetBrains and have given them lots of money over the years but I prefer Ceylon to Kotlin. They are very similar languages but I just like the Ceylon syntax more.
""" If a grand unified theory of programming language existed, its implementation would be called Scala """