Plausible

Quests and Missions should be diverse and avoid repetition. Warning: it should be plausible. Why such a person ask for such a Quest or Mission?

Quests and Missions are showed in a plausible form to the player: a Murder Mission will not be asked like this

Robert: "Murder mission against Raoul the baker"

but more like this

Robert: "Raoul the baker slept with my wife. Make him dead."

or like this

Robert: "I want Raoul the baker is dead before the sunset.
   My reasons are private."

A Quest composed of a Murder Mission, a Kidnapping Mission, a Recovery Mission and a other Murder Mission will not be proposed like this

Sedecias: "Kill Raymonde. Kidnap Gertrude. Ask 10,000 gold pieces
  to Raoul. Kill Gertrude."

but more like this

Sedecias: "I hate the Raoul the baker success. I want to make him
  suffer. *Niark* *Niark*. Kill his wife Raymonde, kidnap his daughter Gertrude,
  ask 10,000 gold pieces as ransom and then kill Gertrude. *Niark* *Niark*"

In the previous example, the quester know by advance all of the Quest. It's also possible to give information to quester only progressively. Information can be given by the Quest giver or by an other person.

   Raoul: "My wife Raymonde has disappeared, help me to get her back.
           I suggest you begin with Sedecias. He has always hated me."
   [The player goes to Sedecias...]
   Sedecias: "I know where Raymonde is but I'll tell it to you only
              if you get me back a shark teeth."
   [The player comes back with a shark teeth]
   Sedecias: "Damn', you succeed..."
   [Sedecias throw negligently the shark teeth.]
   Sedecias: "All the same, I'll keep my promise: take this letter.
              I stole it in the Raoul mailbox."
   [Sedecias gives the letter to the player who reads it.]
   "Dear Raoul,
        I feel tired, so I decided to go one week in thalasso. I'm sorry
   not to have told it to you, but you were working and my friend Denise
   had just given her place to me, because she can't use it(...)."
   [The player goes to the thalasso centre]
   Receptionist: "Sorry I can't give you our clients name.
                   But you can work here today, you could meet them.
                   The pay is 3 silver pieces a day.
   [The player works but don't find Raymonde.]
   [He decides to meet Denise.]
   Player: "Where is Raymonde?"
   Denise: "Ha, ha. Too late! I always hated Raymonde since she won
   the applepie contest. Now, I'll have my revenge. I put her in a package
   and send her to Klouguirec, and no-one could prevent me from winning the
   applepie contest next week. Understand that I can't let you go now you
   know the truth, you'll die!"
   [Denise attacks the player.]
   [The player kill Denise and go back to Raoul.]
   Raoul: "Thanks a lot. Now I know Raymonde is at Klouguirec, I'm better.
           I'll go immediately to recover her. As a reward, my bread is
           free for you for one year."
  

It brings a dynamical and non linear aspect to the Quest: if the player would have been killed by Denise, Raoul would have sent an other quester; the player could have killed Sedecias because he laughed at him; the player could have lied to Raoul, done no research and said his wife was at Schmuziskthan; the reward given by Raoul wasn't determined by advance, it can be function of time spent to complete the Quests or of some others factors; etc.

Events before the Quest could have never existed, as the stolen letter (even if it is always possible such events happened without witnesses). But the Quest could also be based on true events or facts (for example the hate between Raoul and Sedecias).

Future events discovered during the Quest can also happen 'really'. For example, the applepie contest could really be organised (a 'cook the best pie' Quest is possible, with a research of ingredients, cookbook, etc), or player could see Raoul to close his bakery and to go.