Slightly faster processors, usually with around 15-20% improvement over the previous model.
Larger on-board storage capacities.
Increased display brightness. Different colors and materials.
ECG readings (Series 4) A digital compass (Series 5)
International Emergency Calling (Series 5) A better heart rate sensor (Series 6)
A blood oxygen sensor (Series 6) A very slightly larger display (Series 7)
A thermometer that’s only useful for cycle tracking (Series 8)