While Siri sounds decent most of the time, it still comes off as a little robotic. With iOS 13, however, Siri should sound a lot more human. At WWDC, Apple said it's taken a "huge step forward" by incorporating a neural text-to-speech transmitter that makes Siri a lot more natural than before. It has a much more natural cadence, and better emphasis on certain words.
In a demo on stage, Apple showed that the Siri on iOS 12 sometimes hiccuped her way through complex sentences. The version on iOS 13, however, sounded a lot smoother, with much more streamlined transitions and sounded a little less robotic.
Last year, Siri Shortcuts debuted on iOS 12. With iOS 13, a new "My Shortcuts" space will make them much easier to find. Multi-step shortcuts will also apparently be easier to set up.
Apple also mentioned Siri integration into other devices. You can set up EarPods, for example, to announce incoming messages while you're working out. With CarPlay, Siri also now works with third-party apps like Pandora and Waze.