Well, all games should have be something like this :
http://img228.imageshack.us/img228/8327/climax3va.jpg
What im trying to explain ( with my brilliant paint skills ) is that a mission should always have a sort 'tense'. The line goes up up and up , that indacates that the mission is getting more interesting and fun to play. And when the line goes down its supposed to be climax ... like a city-escape or just a 'defeat-end-boss' mission. i've seen a lot of games that just dont have a story-line.
My conclusion is that the missions/quests must have a normall line. Not that when u start suddenly 50 orcs jump on u and when u defeat them u got to walk 10 km till the end