Aggron claims an entire mountain as its own territory. It mercilessly beats up anything that violates its environment. This Pokémon vigilantly patrols its territory at all times.
– Pokémon Omega Ruby Pokédex
Mega Aggron has become pure Steel type instead of Steel/Rock, which I think is a better type defensively. While Rock has some resistances, it also comes with many weaknesses. I guess it lost its Rock typing because its Steel armor covers up most of the rock parts?
It gets the ability Filter, which was previously exclusive to Mr Mime and Mime Jr. It reduces Super Effective moves by 1/4, which makes Mega Aggron even better defensively.
Stat-wise, it gains more Attack and a bit of Sp Defense, but most importantly, its Defense sky rockets to 230 (from 180), making it tied with Mega Steelix and Shuckle as having the highest Defense of all Pokemon.