The 6809 is a powerful 8 bit CPU. In addition to just pushing pixels, the higher level engine is a master class in priorities, tradeoffs to maximize player action, given arguably limited resources. The better features in that CPU were all employed to great effect.

The sound system, plays one sound at a time. The right sounds happen at the right times, conveying a ton with just one sound channel.

Baddies are moved out of the field of play at times to keep frame rates up.

Single buffer, screen draw happening on alternating upper, lower regions to avoid display artifacts, tearing and such.

So many great choices!