Great, memorable, game changing one. For example:
Good idea: An interstellar space traveler with a ship that can move through time and space and adapt its shape to match its surroundings.
Great idea: Having the adaptation ability fail so that it’s stuck in a charmingly idiosyncratic form which stands out from its surroundings.
Take Wolverine: When Len Wein introduced everyone’s favorite Canadian mutant in the pages of the Incredible Hulk, those trademark metal claws were intended as simply part of his costume. But it was Chris Claremont who took it a step further: the claws were in his body. Metal claws as part of your costume: cool. Metal plated claws as part of your skeleton: cool, kind of creepy, and way more interesting.
Or there’s the story of a young boy who finds himself the recipient of a great destiny where he will become the greatest magician of all. But what makes Harry Potter so much more compelling than Tim Hunter?
In Harry Potter, J.K. Rowling makes adolescence part of the journey. Harry’s “Hero’s Quest” begins when he enters wizard school at 10, and along the way he picks up skills and experiences that help him face off against his ultimate nemesis at age 17, where he’s finally old enough to go out on his own. The metaphor would almost be overbearing if it weren’t so neatly interwoven into the story. Is it coincidence? How many people’s teen years didn’t feel like some kind of epic journey where every experience was fresh and bigger than life and the most moving thing to ever happen ever? The Harry Potter books are guideposts through the most harrowing period of most peoples’ lives.
These are just some examples. I may touch on more as they come to me, but it's something I'm sure we all could weigh in on.