Rule of Game Design #1 :
Character - Goal - Obstacle - Solution
Character : it's you, with strengths and weaknesses
Goal : it must be clear and accepted
Obstacle : it must be CLEAR and at the height of the character (not too easy, not too hard). What's at stake must be clearly defined and coherent with the goal.
Solution : it must be CLEAR, logic and coherent with what's at stake.
Good example :
Bob the fighter is picking his nose in town. Joe the mage tells him that the demons attack his home town, so he must kill them before they destroy it.
Bob's goal is to protect the town, it's clear and he accepts it.
The obstacle is the demons. Clear, and what's at stake is his home town. It's big enough to interrest Bob, and clearly in the field of capacity of demons. Bob's death is not really what's at stake, because he can be resurrected. IT only slows him down.
The solution is clear : kill the demons. It means there's a finite number of demons and when they'll be dead, it will be over, the home town of Bob will be safe.
Bad exemple :
Bob's picking his nose again. Joe the mad alchemist arrives in town. He mutters something about evil forces and purple bunnies, then he vanishes. Demons appear, followed by orcs, purple bunnies and meers. They all attack Bob. Bob manages to kill two orcs and three bunnies before fleeing. There's nothing on their corpse. Three hours later, all the monsters disappeared and all the roofs are now dryad green.
Later on a website the player reads that he had killed the orcs only, the roofs would have stayed the same, and if he had killed the purple bunnies in addition, the river crossing the town would have become neon purple.
Neither the goal, the obstacle or the solution are clearly defined. The player doesn't know anything and all what he saw made no sense to him. What was at stake is totally ridiculous compared to what was the obstacle. Result : he's frustrated and shrugs.