Quests are missions that are given out by survivors or triggered by visiting different locations and picking up quest items in Dead Island and Dead Island: Riptide. After completing the campaign, the XP reward changes and it's based on the player's level. For example every continuous quest will give 0,5% of required XP to level up (except for "Time to Booze" and "Blood Compound" which will give 1,5% and 1,33% respectively), example: A level 57 player (6958069 - 6123454 XP) will receive the difference between the XP that will be needed to level up (in this case "6958069") minus the XP that was needed to reach the current level (in this case "6123454"). The quest reward will be based on this result ("834615" in this case), and you apply the percentage of the quest and you will have the XP reward. The following formula is how to calculate XP reward:
(XP needed for next level) - (XP needed for current level)] * (XP multiplayer)
Main and side quests do not to have a concert formula for calculating these results since the difficulty does no appears to have a fixed XP multiplayer, but the XP reward is still based on the player's level.
One quest was cut from the final game. We know of the quest giver and broadly what it would entail from start to finish. However, we don't know of the name of the quest or where it would have been given in the game. View the quest here: Cut Quest.
Hidden Quests[]
There are a couple of quests that are completable but don't show up on the quests list in the menu or have a quest popup on the screen. They are usually only available when a certain requirement is met, but they are normal quests in all other regards, with a final reward and only being completable once per playthrough. See the quests here: Hidden Quests.
The difference between side quests and main quests is at time blurred. There are some side quests that are really part of main quests and must be completed for the main quest to be advanced or completed. In some cases why this was done is unclear (e.g., Locked Armory) while others make more sense (e.g., Ashes to Ashes and On the Air).
Some of the continuous events actually have an end. When the Laboratory is overrun at the beginning of No Time to Talk those quests will no longer be available. This applies to Blood Compound.
Civilian recue quests are a new type of quest in Dead Island: Riptide. As you explore the world, you will find a random survivor trapped by some zombies. You can save them for some XP and Cash. Later they can usually be found at a nearby safehouse with other survivors. They are always easy difficulty and always reward you in cash.
Riptide also has some special quests which are known as Team Quests. These are independent quests that can be activated from the very start of the quest, but you don't talk to the NPC to start them. Instead, you start them from their own 'Team Quest' header in the quest menu. There are three tiers of each quest and each one is harder than the last. If you do a team quest for a merchant, they will get better wares and better prices the higher the quest level, while completing it for a hero means they will wield better weapons during a Siege Event.