I’ve been using the Release Candidates for a month or so now. Favorites features so far:

• Traits with parameters. Haven’t figured out yet, why abstract classes are still needed, but not having to change trait to abstract class just for introducing parameters definitely feels right.
• Powerful Enums. They are now Algebraic Data Types, so can carry different data within each variant, as one might know it from Rust. It’s also just nice, because in Scala 2, enums always felt awkward to use.
• export keyword. Allows you to make something that you’ve imported available as if it were defined in your scope. Particularly useful for creating preludes. Again, for the Rust devs, this is similar to pub use.