My feeling on this is either go full monty or don't bother. Very few games ever have a fulfilling story. If it doesn't have intrinsic reward, like talking to an NPC you want to know more about, don't go on endlessly about the desperate need to retrieve the mcguffin/fulfill the prophecy.
There is also a really basic fail in the way many assassinations were designed in the first games. You would get a 'mission failed' just for being discovered. That's not a game stopper, that's a result, now show me the consequences, don't quit just after it got interesting.