A11, wireless charging, improved cameras.
The "S" cycles have always been boring and the sales numbers support that.
I would actually say that "s" versions offer some of the biggest improvements over their predecessors, but you're right, they generally don't get the same hype because they look the same. For instance, the 5s compared to the 5 introduced TouchID, and a 64-bit processor. The 6 is really just a slightly faster 5s with a larger display. The 6s introduced faster TouchID, a much faster processor, double the RAM, and live photos. The most notable features of the 7 are a better camera and the lack of a headphone jack, which many see as a disadvantage.