But MP isn't a slight improvement, that's the thing. If it was just Metroid Prime 3: Now with same Gamecube controls then it wouldn't be anything special. Instead, it's Metriod Prime 3: With brand new Wii controls that worked wonderfully and one that nobody has ever done before so yes, the game is special.
As for Halo, it was special for two reasons 1) like you said, the system link ans 2) because it revolutionized the way FPS console games played. If a PC game did something that made a complete new way of playing due to its controls, then yes, it would get recognized. Metroid Prime isn't just slightly improving the controls at all, it's making a brand new control scheme that just works really well. A game that just slightly improved the control scheme would be something like RE4: Wii Edition therefore not special in a revolutionary sense (though special in that it made that game better).
