One common complaint around MMOs is that the quests are all the same. You start off doing one or two types of quests and, by the endgame, you might be doing the same kinds of quests but on a larger scale. The reason behind this isn't that MMO developers are uncreative with their question design, it's because there really are only a small number of quest types that can be created, regardless of window dressing. Much like The Hero with A Thousand Faces boils down centuries of human storytelling to show that a lot of common structures exist, MMO quest design is limited by the small number of tasks you can assign players.
These quest types are:
- Kill X of Y: You have to defeat a certain number of the same opponent. This starts out as Kill Ten Rats and ends up at Kill 250 Elder Rat Gods.
- Kill [Named] Y: Some mob has gotten tough enough to earn itself a name. You are to assassinate it for rising above its station.
- Delivery (aka Fedex): The quest giver wants something delivered to someone else. In a time of magic or ultra-tech science, the best way of getting it there is to give it to some wandering adventurer and have them deliver it in person.
- Collect X of Y: A character is tasked with finding a certain number of objects of a certain type to continue the quest. This starts out as Collect 10 Rat Droppings and ends up at Collect 100 Elder Rat God Droppings. Sometimes it involves collecting the organs of creatures you kill e.g. collect 10 Rat Noses - please note that not every Rat will have a nose to collect.
- Escort: Lead another NPC from point A to point B. There will always be complications, especially if the NPC decides that they'd rather run straight through the most dangerous part of the zone, insulting its inhabitants and frequently get stuck on rocks / logs / their own shoes.
- Locate: A particular individiual, item or location needs to be found. Sometimes they will be marked on the map, which defies logic since they are already located, but is a lot more fun than wandering aimlessly looking for them / it, only to find out they were / it was around the corner from the quest giver the whole time.
- Defend: Defend an NPC, item or place for a fixed period of time against waves of attackers. These kinds of quests often guarantee that players are forced into one location for the entire time until the quest is complete; it also sees them get very aggravated when they fail the mission on the last wave.
- Interact: Slightly different to Locate in that the specified NPC / item needs to be activated by the player as part of the mission requirement. This means it involves at least one extra click. Most puzzle-solving quests (i.e. put the pillars in the right spots, align the coloured pegs, etc) are Interact quests.
- Craftskills: In order to complete the mission something needs to be crafted by the player (or developed by some kind of craftskill). Every now and again the mission designers remember there are other systems in their MMO outside of combat and movement and drop one of these in.
- Reach Achievement X: The quest is to reach a certain achievement, such as a particular level or craftskill rank or even earn a particular achievement before you can continue. Usually this is used as a gating mechanism - the contact won't even talk to you before you reach this particular achievement - but sometimes the quest itself states what achievement you have to get to before continuing.
(I think this just about covers it - please let me know if I've forgotten any.)
There are certain twists that can be added to these - you have a time limit, it is a combination of things (Kill 10 Ubermensch and Collect 10 copies of Thus Spake Zarathustra, Escort Sir Deathwish to Point Unexplored and then Defend him for 10 minutes), it can be for solo players or require a group. But these are the basic quest types.
How they become interesting is how they are presented and what happens along the way. After all The Lord of the Rings can be broken down to "Deliver the One Ring to Mount Doom", but there is a lot more to the story than that. Unfortunately for most MMOs the quest objective becomes the point, not the motivator, for the narrative at hand.
EDIT: Added in Craftskills as a quest type (about 6 hours after originally posting).
EDIT 2: Added in Reach Achievement X as a quest type and added a bit about puzzle-solving quests to Interact (about 18 hours after originally posting).