Outward comparison of the Series 6 (right) with its one-year-old sibling (left) reveals only subtle differences, but that's what teardowns are for. These details we already know:

LTPO OLED Retina display optimized for always-on functionality—this time without Force Touch

64-bit dual-core Apple S6 SiP (System in Package)

Updated sensor array measuring heart rate, ECG, and now blood oxygen levels

Compass and realtime altimeter

Water resistance to a depth of 50 meters