The breaking changes have been minimal since I've started using it. Typically just slight changes to some type signatures. The core parts (Actors, Reference Capabilities, Classes/Traits/Interfaces) are all stable at this point. I'm amazed by how well ponyc (the compiler) works. You just run it and it figures everything out.

If I could change anything in the language right now, I wouldn't mind seeing a couple of the reference capabilities renamed (trn, box, and maybe tag specifically), though I don't have any better names to offer. I'd also like if Actor Behaviour dispatch had a different syntax than Object method invoke.