I think the main difference is Java interop. Kotlin is fully compatible with Java and Java libraries, while Ceylon is not quite (the Ceylon FAQ says: "since Ceylon will be based on its own modular SDK, making a clean break from the legacy Java SDK, Ceylon will require new frameworks designed especially for Ceylon").
Well, this is a sensitive subject. Scala is no doubt more powerful and has some awesome features, but I'd say that Kotlin has 99% of the features 99% of Scala developers choose Scala for in the first place (in other words, those Scala features that Kotlin is missing are not the ones that made most Scala developers choose Scala), while it is far easier to learn (in its entirety). Also, IDE support is far superior, and will remain so (because some Scala features, like structural types, make some actions like refactoring impossible at times). Also, it compiles to Javascript as well as to Java bytecode.
Sorry I wasn't more specific. How does it compare with Scala's Java compatibility? I know that Java compatibility is one of Scala's strengths, but the more I read, it also seems like it's one of its biggest weaknesses.
Sometimes I just think Scala sucks at marketing – completely.
- I think that especially the adjustments and features in the upcoming Scala 2.10 address real-world requirements like performance, reflection and database access.
- Scala compiles to JavaScript for months already as well as .NET.
- I remember the claim regarding IDE support from the Kotlin presentation. JetBrain's own engineers disagreed with it. I think the jury is still out there.
