There's a beautiful simplicity to it. The shots are all fair, but challenging. The game's progression is very simple, but there are options on how to approach it (do I focus on getting to Total Annihilation first?, do I wait for MB until 3rd attack wave or 4th? 5th?, when do I collect my Stroke of Luck shots?, and so on). The sound, music, and art are all best in the business.
For competition, few games offer such great balance. AFM is challenging, but never cheap. It can be brutally hard, but control is always yours.
I can't really find flaws when I examine AFM. It's just rock solid.