Honestly the main thing I find killer about Kotlin over Java is its stricter null safety (types do not permit null values unless you mark the type as nullable with a "?" suffix). (I know Java does have the @Nonnull annotation but it's extremely verbose to use much and Java doesn't enforce its use on values that are expected to be non-null, which both greatly diminish its usefulness.) I've been spoiled by languages like Typescript and Rust which have great null safety like Kotlin, and it's very annoying going back to a language without it and questioning whether a null is expected or possible for every single value.
I wish I could get a strong sense of the hype around null safety. Sure, I would like to have the extra expressive option. However, optional has been fine for me. I still barely use it. It has to be at least 3-4 years since I last encountered a null pointer exception in my daily java programming. I feel like there are plenty of other things that pain me on a daily basis that rank far higher on my asks for the language. Collection and stream ergonomics cause constant pain, for example.
