8. It has an exceptional supporting cast.
Thematically, Sons mainly depends on the performances of the core three characters: Jax (Charlie Hunnam), his antagonistic stepfather Clay (Ron Perlman), and his scheming mother/club matriarch Gemma (Katey Sagal). Technically speaking, that main element is so strong that the series could employ real life-bikers and Sutter's second-cousins and the show would still thrive. We've seen plenty of dramas put the heavy lifting on their lead/the A-story only to surround him with a weak supporting cast that only exists to give him a break in between scenes (ahem, Dexter).
One of the main reasons SoA is great instead of just good is because the supporting cast is filled with actors just as skilled as the main three. They've all elevated characters that only sometimes get their own arcs into fan-favorites. For example, Juice's (Theo Rossi) informant arc and subsequent suicidal spiral in season 4 was the character's first time in the spotlight, yet viewers were already invested because they'd grown to appreciate him by then. And you'd think Opie (Ryan Hurst) was the main protagonist himself from the way his brutal in-jail murder shook fans up earlier this season.