Almost every ability each of the main three classes (Engineer, Adept, Soldier) is tied to an Achievement. Each weapon achievement requires 150 kills to unlock. Tech and biotic achievements require 75 successful uses of the skill to unlock. Some offensive abilities, like Singularity, and some defensive abilities, like Barrier, can be used at any time, but you are only able to gain the respective achievements using your main character, not with your party members.