All attacks now have a chance to critical hit. However, they don't do more damage. Instead, they apply the weapon equivalent of elemental status effects: knockbacks, stuns, bleeds.
High Aim characters have a greater chance to land critical hits with attacks. 2 points = 1% more chance. 100 Aim = 50% greater critical hit rate. (so example a 10% crit weapon would crit 15% of the time).