The Expert's Reward is a Cooperative Mission reward given to players who meet the "Expert" completion criteria for a mission for the first time. It always grants 500 more XP than the Standard Reward and a varying amount of gold.
Many Expert's Rewards are attained by completing a mission within a specific timespan. However, for some missions such as Gyala Hatchery and the Eternal Grove in Factions and most of the Nightfall missions, the criteria can be different.