It added type checking ala. Hack and Typescript. It's optional, and doesn't give you the same compile-time guarantees that a language with pure static typing does.

Also, dynamic languages like Groovy take a performance hit. Static typed JVM bytecode runs faster than dynamic bytecode.

Whether either of those are relevant to your application really depends, but Kotlin definitely is going to gain some followers due to it's differences from Groovy.

I'm learning Swift right now, and really appreciate type checking.