Important note: it's the preview release. Not everything will be done, there will be gaps. But the first parts of the work have already merged to master, and it's coming.
* Its interfaces are very awkward. There are superior implementations of what they tried to do in other languages.
* Subpar IDE experience, made worse by how interfaces are implemented. You need an IDE for any non-trivial project.
* Very awkward choice to have visibility be determined by the case of the first letter in the function or variable. Simple changing of visibility to public will require larger diffs if it's used a lot.
* No private visibility, only package private.
* Very crude import mechanism. Try to rename a package and see how the IDE fails to figure out what to change.