Oh I agree. There's a difference between taking a fic IDEA and moving further with it and expanding on it by changing the characters/etc, but in that process I would also hope that the new characters' reactions to things would differ as well, especially if it was something emotionally heavy where not everyone's going to react the same.

I've often taken one idea or 'plot point' and looked at it from the point of view of several different sets of characters, just to see how they'd react to that situation. If they all reacted the same though, I'd consider it bad writing on my part.