Kotlin allows implementations to be delegated via delegation pattern that replaces implementation inheritance with zero boilerplate code.  A derived class can implement an interface by delegating all of its public members to a specified object. This is independent from overrides.
Type inference for variables and property types is automatic. New symbols, methods, keywords and constants make it very easy to declare and use variables.
Slight modification of a class does not require a new subclass. Instead, we can use object expressions and object declarations.  Object expressions take an object parameter of an anonymous class usually derived from some type or types and overrides the methods associated with that type or types. Object declarations are used with singletons where the declaration is much simpler than in other languages. It uses the object keyword followed by the class name and the implementation.
Kotlin allows writing a companion object inside the class so that its members can be accessed using only the class name as a qualifier. Companion object is useful when we need to write a function without instantiating a class but has access to the internals of a class such as a factory method. It makes use of the object declaration syntax with the companion keyword. They look like static methods but are really instance methods and can implement interfaces. It is resolved when the corresponding class is loaded which is typical of lazy initialization for object declarations as opposed to immediate initialization for object expressions.
Classes that are used exclusively for data are called data classes and declared with the data keyword.
The compiler automatically creates methods such as equals(), hashcode(), toString(), copy(). These are formed based on the type parameters in the constructor. These classes come with a few restrictions such as they cannot be abstract, sealed or inner but some of these were even relaxed in versions subsequent to 1.1
The standard library provides Pair and Triple as data classes.
Kotlin is perhaps the first to provide a clean separation between readonly and mutable collections. The readonly provides an interface to the collection to access the elements of items. The mutable interface extends the read only interface with write access. This is makes it clearer to call out collections that are meant for reporting stacks and do not interfere with the operations ongoing with the existing collection.
Kotlin is therefore, more than a notation change from Java. It packs features that were not seen earlier with Java.