Significant new features include &., Array#dig, Hash#dig and the very user-friendly “did you mean” when you get the name of a gem wrong when bundling your gems.
Of course there are performance improvements, but this blog post brilliantly walks through all the new cool stuff in Ruby 2.3.0 … bookmark away!
Also worthy to note that Heroku currently supports Ruby 2.3.0 but is running a preview version, I’d expect an update to the final release before the end of the year