Armor is divided by light, medium and heavy with their own pros and cons so that players actually have a reason to use either one. The lightest armor allows you to move really fast, sprint for long periods and melee faster but provides less armor protection. The heaviest armor provides the most protection but you move slower, melee slower, evading has a delay and can't sprint as long.
How about...
-- Armor weight not class-restricted. (Adepts, Engineers able to wear heavy if desired).
-- Could mix different weights on the same character, e.g., heavy chestplate, light gauntlets.
-- Armor weight impacts power cooldowns, similar to the weapon weights in ME3.
-- Ditto shielding. More shields add weight, and impact power cooldowns.
-- Heavier armors provide better protection against melee and explosions, and put more mass behind your melee attacks.
-- Lighter armors could be partially or fully self-healing. Heavy pieces damaged in battle would need to be repaired at the armory.
That last bit implies an actual armor layer. All we've seen thus far for the player are shield/barrier and health.
A player could load up with heavy armors and shields, and become a melee monster, forgoing power use. Or possibly be more of a tank, like ME1's soldier w/ immunity.