kotlin looks cool. But it's missing some minor things that would make it much more friendly:
* no regex literals
* a little verbose(instead of [] for arrays, it's "Arrays")
* hashes IMO should be JSON
* why doesn't null evaluate to false? God I hate writing if (a != null). I'm much happier in javascript/ruby/groovy land where if can type "if (!a)" or "unless a" or even cooler "raise Exception("a should not be null") unless a"
* why did the world standardize on camelCase instead of snake_case ? the world was wrong about this!
basically a little more javascript and ruby influence would have been good. make the language more "fun" and expressive. There is no equiv of the ||= operator or | for joining arrays, etc
But the big things it's missing are more ecosystem features:
* REPL
* an alternative build system (please don't make anyone use maven!) should be an interpreted scripting language. I'd look at Rake or Gradle for inspiration
* rails like web framework