Notably: "The main intent of introducing proto3 is to clean up protobuf before pushing the language as the foundation of Google's new API platform. In proto3, the language is simplified, both for ease of use and to make it available in a wider range of programming languages. "

protobuf has been great for private data storage and RPCs, but it hasn't really been used as a schema language for public APIs until now. I have written my own JSON <-> protobuf codecs in the past and had great results (my guess was that Google did something similar internally). But this would be even more useful if this were widely used (better tooling), and some of the changes they're making around adding more types would also be helpful, so I'm excited to see this.