Personally, I am most excited about the ability to write Java-compatible enums! Just extend java.lang.Enum, and it works!
enum A extends java.lang.Enum[A] {
case MONDAY, TUESDAY, SATURDAY
}
enum B(val gravity: Double) extends java.lang.Enum[B] {
case EARTH extends B(9.8)
case JUPITER extends B(100)
case MOON extends B(4.3)
case Foo extends B(10)
}
See the test cases here: https://github.com/lampepfl/dotty/tree/master/tests/run/enum-java