Functional/SAM interfaces (transforming interfaces with a single method into a simple lambda), pleasant usage of lambdas (last lambda parameter can be put outsi

Functional/SAM interfaces (transforming interfaces with a single method into a simple lambda), pleasant usage of lambdas (last lambda parameter can be put outside of the call like(this) { ... }, leading to a language that lends itself really well to building a DSL, coroutines were not a low hanging fruit but absolutely are infinitely more pleasant to use than RxJava, operator overload including invoke(), ranges that are pleasant to use (0 .. 10).forEach { }, or when (floatVariable) { in 0.0f .. 1.0f -> ... }, pattern-ish matching with when (not quite full on functional language powerful, and I believe that Java is not only catching up to it but making their switch quite a bit better), a standard lib that is packed full of extremely useful and consistently named methods, extension functions, delegation (if you inherited a SDK that has a piss poor interface, you can simply make a SDKWrapper(val internalSdk: SDK): SDK by internalSdk, which means that it will automatically implement it, and you can then have your wrapper do whatever around it (logging, better functions, DI, etc.))) Kotlin is truly a pleasant language, both when you don't know it (although it can look a bit symbol soup-y at times for juniors), and when you fully know it.