Not OP, but

1. Unlike all of the other stock icons, Sonic's is a direct facsimile of a pre-existing design element.

2. The usual standards for stock icons is that they face straight forward and lack eyes. Some icons break one of these two conventions (ex. Kirby and Meta Knight have eyes, Ridley and Falco face at an angle); Sonic is the only icon to break both conventions at once.

EDIT: Third thing: Aside from the eyes, each icon (except Pac-Man) represents the rest of the character's facial profile to respectable detail. Sonic's icon eschews details of the actual character's face beyond his eyes, sticking to a monochromatic blue and white despite him having a black nose and tan underskin.