What's new:

• Language features
  • Tail-call optimization with [tailRecursive]
  • Constant expression evaluation
  • Suffixes for Longs and Floats: 1L and 1.0F
• Compilation to JavaScript
  • Default arguments
  • Super-constructor calls
  • Class-objects in traits
• Java Inteoperability
  • @ReadOnly and @Mutable for collections
• Android
  • Better annotations for Android SDK
• IDE
  • Support for IntelliJ IDEA 13
  • Find Usages
  • Compiler preferences
  • Smart completion (Ctrl+Shift+Space)
  • Rename from Java
  • IDE warns on misusing Kotlin APIs from Java
• Ant
  • Integration with <javac>
  • Support for compilation to JavaScript

What's new:

• Language features
  • else is not required in when-statements
  • Type argument inference uses smart casts
  • Ability to suppress warnings with [suppress("...")] annotation
  • Local returns in lambda expressions
• Compilation to JavaScript
  • Enums
  • Class Objects
  • Delegated properties
  • Multi-Declarations
  • SourceMaps
• IntelliJ Plugin Improvements
  • Automatic configuration of Maven and Gradle projects
  • Convert member function to extension function
  • Navigation to getters and setters of a delegated property
  • Navigation to iterator/next/hasNext in a for loop
  • Navigation to invoke() from a call site
  • Improvements on Find Usages including support for constructors and overrides

Read more in this blog post

What's new:

• Language features
  • SAM Conversions
  • Annotation improvements
  • Static fields visible from Java
• Android
  • Integration with Gradle for Android
  • Integration with Android Studio
• IDE features
  • Inline variable, Safe delete and other refactorings
  • TestNG support
  • Faster code completion

See a detailed description in this blog post.