Having used plenty of Java in big-ish teams over the years - and mind you, having never used any other JVM languages beyond simple tutorials or scripts - I can imagine that Scala's learning curve might present a problem. Kotlin, on the other hand (as far as I can tell from my admittedly limited experience) adopts many of the best syntax features of alternatives like Scala and Groovy while keeping things simple enough for pretty much any existing Java developer to use right away. On top of that it does not introduce any new APIs or concepts like actors to get a grip on - as such cross-compatibility with existing Java code is 100% out of the box (this was of course a big factor for the JetBrains folks who are using it to develop the future of their IDE platform). And even though Java 8 is a huge advance in terms of functional programming support, it feels like a more natural fit in Kotlin. Essentially I think Kotlin is something like "Java 2.0", or what Java could be if they were willing to throw backwards compatibility out the window.
:) At the same time it's dead simple to learn in its entirety in a short period of time, as opposed to Scala's huge feature set and learning curve. Scala may be the "better" language overall, but Kotlin seems like the more practical choice, especially for teams that have no Scala experience as of yet.