GP&B
Ike 'n' Ike
It determines at what point a character goes into grounded state from the air. In Melee, it's at the shins; in Brawl, it's at the feet. Why this is such a critical aspect is because it allows for a huge number of aerial-based numbers to work properly or even be possible to perform. Brawl landing detection makes it hard to SHDF with Marth, impossible to SH Bair waveland with Mario, doesn't allow Falcon and Ganondorf to stomp through thin ground and platforms, limits how low Fox and Falco can cancel a laser to the ground, the timing for Samus' missile canceling, makes full wavedash length easy to achieve, changes L-cancel timing, and most importantly, requires wavelanding to be a bit higher above platforms to do.