I like Scala and didn't personally find it that hard to start using it but I'm not optimistic about its future as a mainstream alternative to Java. It's just too big and complex and (apparently) too GC heavy to be useful on what is quickly becoming one of the most important Java environments (Android).
My money is on Jetbrains' Kotlin as a significant but manageable step forward from Java. Given its pedigree I expect it to have tooling support that no other JVM language can match.
Part of Kotlin's pedigree are some ex-developers of Groovy, i.e. James Strachan (Groovy's creator) and Alex Tkachman (creator of Groovy++, the inspiration for Groovy's static type-checking). They bring a deep knowledge of implementation pitfalls that can only be gained from doing it wrong the first time.
Of course, Scala creator Martin Odersky completely rewrote Scala after version 1.0, and even in version 2.x, has considered fixing stuff between point releases to be more important than backwards compatibility, so Scala similarly ought to be quite polished by now (version 2.10), in both feature set and implementation.
I didn't know that the Groovy folks were involved in Kotlin. That's great to hear. I think this team has exactly the right experience to build a potential Java killer.
my little worry about scala is that it _keeps getting_ bigger and more complex.
2.9 manage to not change the language much, AFAIR, but now 2.10 gets macros[0], string interpolation, implicit classes, value classes, and a whole new concurrency framework.
All cool stuff, but the nonchalance in adding more and more things makes me uncomfortable.
My money is on Jetbrains' Kotlin as a significant but manageable step forward from Java. Given its pedigree I expect it to have tooling support that no other JVM language can match.
http://kotlin.jetbrains.org/