Performance-wise, 1.3 is the biggest Opus update so far. Several commenters have suggested it should have been named 2.0, but full interoperability is important, and '2.0' could suggest we broke compatibility somehow (we haven't).
Opus 1.3 adds full Ambisonics surround support, improves the built in speech/music detection, and greatly improves low bitrate performance. Wideband speech now goes down to 9kbps, narrowband to 5kpbs, and stereo performance improves especially in the 24-32kbps range.
Go forth and deploy!