1. Kotlin issues:

- Libraries that use reflection (such as Gson) can break some assumptions that Kotlin makes (for example, a read-only field may actually change). There's maybe 2 or 3 little things like that that may catch you by surprise, but once you figure them out it's easy to work around them.

- Stack traces in lambdas are obscure sometimes. Sometimes you won't get the line number.

- Proguard integration has proven challenging.

2. We started from Kotlin.

3. Learning Clojure and Haskell has made me yearn for more functional languages and better type systems. Kotlin has that, in a neat syntax that's very beginners friendly.

I'm firmly convinced that a good programming language needs:

- Strong library ecosystem

- Static type system + type inference. (I won't get into the static/dynamic argument here. Suffice to say that there's a whole class of bugs that you can avoid with static typing, and type inference makes the "but I hate these types cluttering my code" argument irrelevant)

- Killer IDE support (essential for productivity. If you're writing a large application without using an IDE that gives you code inspection and navigation, you're wasting your time)

Kotlin has these 3 things.

4. Top improvement: can't wait for Kotlin 1.1 to be released, and start using coroutines. Other than that, I'm happy with it.