What mechanics, stories or games in their entirety do you think are a nice idea but executed as successfully as a drunk fat man walking along a tight rope?
Although admittedly it isn' t executed particulary badly, I' d still have to say the live ammo in Oddworld: Strangers Wrath. In my view this could have been pulled off so much better, giving more incentive to use particular ammo.
This is all I can think of right now, but I' ll be sure to add to it if anything comes to mind.
Come on, tell me yours