Dunno how or why this popped into my mind, but here goes.
I'm sure plenty of people here have played Adventure (the Atari 2600 game) or maybe even it's modern-incarnate predecessor, Adventure II. I'm also sure there's plenty more people here who fall into the category of "should" play it. You can play it in Flash here, in case you're one of those.
The concept is really straightforward. Sword beats dragon. Keys beat doors. Magnet pulls objects. Ladder lets you scale over/through walls. Get to the end and win. And then there's the twist, and the point of this thread. THE BAT. It makes the game, and yet it breaks the game. Or vice versa. Either way, it certainly wouldn't be what it is if it weren't for that little stinker.
Conceptually speaking, what do you think/how do you feel about THE BAT? Could it make some games better? Would it make games worse? Is it a concept worth exploring to you, design-wise?