Lots of accumulated changes. Debugging and tracing is much easier, commands can have multiple sets of input and output files, rules can depend on single output files from other rules instead of all of them, template expansion handles nested configs, template expansion failure is not an error (makes nested configs work better), other stuff I can't remember.
Not backwards-compatible with earlier versions, but transitioning to v020 should be straightforward.
Documentation and tutorials are out of date.