I would say, obscure unlockables.
I don't mind when games reward you for thinking on your feet. I do mind, however, when games drop absolutely no hints about certain things, then expect you to do something ridiculous or counter-intuitive in the hopes that it might lead to some kind of reward.
For example, imagine you're partway through a game and you meet an NPC who says, "Oh, if only Otto hadn't gone to Ironforge, he wouldn't have fallen in the river. Why did such a fine blacksmith have to die so young?" And you think to yourself, "Didn't I meet a blacksmith named Otto before? Yeah! And he said something about his trip into town, and there was that other guy who said the bridge was shaky..." So, on your next playthrough, you make sure to sabotage Otto's wagon before he leaves. Then, when you're passing though Ravendale later on, "Look, it's Otto!" Apparently while he was repairing his wagon, the bridge to Ironforge fell through. Boy, he sure is lucky his wagon broke, or he might have fallen in the river! And then you check out the items he's selling and, "Wow! A really awesome item that you can't get anywhere else!" Drop some hints, let them figure it out, reward.
Now, imagine if Otto never hinted that he was going into town, and that the bridge was never brought up in conversation, and that nobody mentions anything about him after the fact. Suddenly, if you want that "really awesome item you can't get anywhere else", you have to; sabotage some random guy's wagon for absolutely no reason, go along on your merry way, notice said random guy somewhere down the road, listen as he regales you with some out-of-the-blue story about his mysterious good fortune, and THEN you understand why your random act of cruelty somehow led to a reward. And why did you do all this in the first place? Because you looked up a guide, and it told you to.