It's hard for casual fans, because usually "good defense" means something like Milwaukee's (great paint protection, vulnerable to 3-point shooting, have to turn it up in the playoffs) or Miami's (try hard all season, not much paint protection, good at shutting down from the perimeter and helping).
Usually there isn't that big a difference between "great defense" and "good defense that can turn it up in the playoffs."
With the Celtics, they protect the paint, shut down the perimeter, can switch 1-5 with no weaknesses, help and rotate better than anyone, can play huge or small/fast, and are really physical. It's an absurd combination, and the only thing that can stop it is health.