Marth has always been a well designed and well balanced character. He didn’t need to change very much so he didn’t.
Marth did see a few smoothing tweaks to a few moves, but the real story with Marth is how everyone else changed. Some of the worse characters saw improvements so they could handle his powerful game, but at the same time, Marth is an overall winner because Meta Knight and friends, who were previously such big worries for him, are much more down to earth.
Balanced Brawl Bonus: No Class Change
Marth remains very similar to how he has always been while a certain masked swordsman who competed for his role in the metagame is clearly worse.
Other Major Changes
Forward aerial does less damage.
Full Technical Change List
Down tilt hitbox 0-2 properties: SDI Multiplier (1.00 -> 1.30)
