Bonus

Prophecies campaign
Every post-searing Cooperative Mission in the Prophecies campaign has a Bonus Mission. The bonus is added to your quest log when you aquire it. Bonus mission objectives are obtained either at the start of the mission or while playing through the mission, but it is not guaranteed that your will find the bonus. It's quite possible to finish many missions without knowing what the bonus was, unless you search the mission thoroughly. You get 1000 experience when you complete a bonus, and sometimes receive additional rewards.

You can earn the title Protector of Tyria by completing all of the missions and their bonus missions in Tyria.

Factions campaign
Canthan Cooperative Missions have two possible bonuses. Expert bonuses give you an extra 1,500 XP, a skill point, and 150 gold in addition to the normal reward. Master bonuses give you an extra 2,000 XP, a skill point, and 200 gold in addition to the normal and expert bonus. For all but two of the missions, the bonuses are rewarded according to completion time of the mission. For the Gyala Hatchery (Mission) and The Eternal Grove (Mission), the bonuses are dependent upon keeping NPCs alive throughout the mission.

You can earn the title Protector of Cantha by completing all of the missions with a Master's Reward.

Tips
A lower level character receives more experience from kills, so you will gain less experience from monsters if you use the bonuses to achieve new levels. You could maximise the benefit of the extra experience by completing the mission bonuses after reaching level 20.