I'm really happy about grouped notifications. It is something I missed from Android and it finally makes it to iOS.
And FaceTime with multiple users is a great addition, not something I'll use daily but sometimes it will be funny to have a call with all the grandparents, I wonder how my baby will react to that.
Speaking of grandparents they all have old devices like 5s, 6 and 6s so performance improvements are more than welcome! I have an 8+ and plan to upgrade to an X like model this year so I never feel performance problems, but many of my relatives do and they complain about that. This is maybe the best feature at all, and if it really delivers those enhancements on old devices it will be the best iOS release in years.
As a developer I'm thrilled about Core ML 2 and its ability to reduce the size of models we have to import, it means apps using Machine Learning will be faster and smaller in size, great news! Even the ability to save and share AR scenes is really interesting, I have some apps in the work and plan to take advantage of some of the features announced yesterday.
In the end I can say I'm happy about iOS 12, both as a user and a developer. Sure not everything on my wish list has made it to 12, but there are some good enhancements.
I think people expected more from Siri, we have the possibility to customise it with our own trigger phrases now, but it isn't a real improvement in Siri, it doesn't seem it will understand us better especially if we ask something outside the custom phrases or the usual stuff (Play this song, send this message, take me to that place etc.)
Apple want to protect our privacy, machine learning works locally on the device so they cannot take advantage of their millions users to improve their algorithms the way Google does. Sure some Siri failures could be addressed even without compromising privacy, Siri sometimes is too dumb, but Google and Amazon will always have an advantage.