Here's my list:
- It scales very well from level 1-20. The math just works at all levels of play.
- 3 action rounds
- Encounter design and balancing is easy for the busy GM
- All of the classes are good, flavorful, and have interesting options
- Teamwork is highly encouraged through class and ability design
- Martial/Caster/Support balance
- Degrees of success/failure
- Easy, free access to the rules
- The ORC license
- https://pathbuilder2e.com/
- Pathfinder Society Organized play is well done and well supported by Paizo
- The rune system for magic weapons/armor