Have you tried demos of story-driven games? Like adventure games, pseudo rpg's or visual novels.
I have, it was called Sage Fusion couple years ago. I've completed the demo which as you've guessed ended in cliffhanger. I have to buy the sequel to get the complete the story.
I was curious of the story of course HOWEVER there are couple things about the demo which put me off from getting the sequel.
1. I'm not fond of gameplay the demo offers. It's mash up of point n click and turn based RPG BTW. Neither element are deep enough IMO; point n click is kinda shallow and the RPG doesn't have what I'd expect from an RPG.
2. The final ... part of the demo is too linear for me which gives me doubts of the ... game's quality.
3. The maximum party size is 3 which is below my minimum limit for party size which is 4. To make matters worse, due to story, most of the time in the demo, I only have 2 people in the team :P.
Okay, maybe you could call that demo as prequel or something but for me it's a demo :).
That reminds me, I have also played a demo of another RPG and demos of some point n click games. Neither of them make me want to get the complete game.
when you finish it make you say "I'll definitely play the final game" and "I don't care about the final game"?
Regardless of genre, the demo must show the basic gameplay of the complete game. The gameplay has to be impressive to convince demo players to play the complete game. Doesn't matter if the game is story driven or not.