A lot of players equip their core skill thinking that it's a passive skill, but it's not... your core skill will give you a buff or reset your cooldowns according to your specialization.
Let's take Mage for example: Fire Mage has a special orange bar right above the mana bar which indicates the number of accumulated power that needs to be consumed in order to gain a critical chance boost and increase your DPS. You need to learn your rotation to increase your "resource" and then consume it, otherwise your DPS will be ineffective.