Ceylon is very similar to Kotlin, but I much prefer Kotlin, mostly because it so seamlessly interoperates with Java (more so than any other JVM language I'm aware of). Also, Kotlin already has kickass IDE support.
But I like Ceylon more because:
* Union and intersection types.
* I find the type system cleaner and more elegant.
* It's an attempt to build a whole new platform with a new SDK. Kotlin is, in a way, just a syntax skin over the Java platform.
* Reified generics. Kotlin got rid of them.
* I personally find Ceylon's "String str" more readable than Kotlin's "str: String".
* Type-safe metaprogramming.
* Comprehensions (not 100% sure Kotlin doesn't have them).
* It's compilable to JS.
* Slightly more regular syntax, more readable generic declarations, better web documentation, currying, spread operator, etc.
Thing is, neither Kotlin nor Ceylon (nor Scala, IMO) add something fundamentally necessary over what's available in Java (8). They mostly provide productivity enhancements. For me, added productivity is really nice, but it doesn't justify switching a language, unless it's a 10x productivity boost (Clojure is different; it provides a whole new way of thinking and managing state and data, which is very important for concurrency - this is big). So in that case, I'd rather give Java a facelift than change to something else altogether. Kotlin is just a Java facelift. It's trivial and transparent to mix Kotlin and Java classes, even in the same package.
I don't understand the Clojure bit - do you mean STM?
As for Clojure, I mean this: http://www.infoq.com/presentations/Value-Identity-State-Rich... (basically: immutability, controlled state transitions via agents/atomic/STM)
_Please_ try Ceylon IDE, and let us know where it's nonasskicking. We've worked very, very hard on it, and we think we have something really useful and usable, but of course we very much welcome feedback.
FTR, I personally pushed almost 240 klocs to this project over the past two years.