Missed one
A major thing they fixed was that the fingerprint sensor now seems to work perfectly with damp fingers. Previously if you, say, washed your hands, it would take a few minutes for your skin to get back to a state where the device would recognize you. I haven't yet been able to trip up the 6S on this - even unlocking immediately after stepping out of a long shower.
The first fingerprint (5S / 6) sensor was very good. The 6S sensor seems to be pretty much perfect.