For me it was when I could send/receive Messages from my Mac and the messages synced with my phone. That was super convenient and I probably compose more messages on my Mac or iPad than on my iPhone. Mirroring is neat but I never use it for anything, but did test it out a couple times.
I just went back to Snow Leopard on Wikipedia and looked up all the new things since then... these are things that made a difference for me: AirDrop, Notification Center, Password App, iCloud Drive, Dark Mode, Activation Lock, Low Power Mode.
Things I miss... Dashboard, Launchpad, iTunes, 32-bit app support.