What Kotlin features do you find most pleasant or attractive for development?

In Kotlin, I especially like the following features:

• Concise syntax — less boilerplate code compared to Java, for example, data class for simple creation of classes with fields.
• Type safety and null-safety — built-in system prevents NullPointerException errors at compile time.
• Extension functions — ability to add new functions to existing classes without inheritance.
• Coroutines support — a convenient and efficient way to work with asynchronous code and threads.
• Interoperability with Java — can use existing Java libraries without issues.

Example of a data class and extension:

data class User(val name: String, val age: Int)

fun User.isAdult() = age >= 18

val user = User("Ivan", 20)
println(user.isAdult()) // true