IMO the best thing she ever did was bring in Dumbledore's backstory in Deathly Hallows. It chilled me when I recently re-read Order and at the end Harry's like 'you don't know how I feel!' and all that. Of course, we now know that Dumbledore knew exactly what Harry was going through. And reading Deathly Hallows knowing that Dumbledore was actually in love with Grindelwald, as JK Rowling confirmed later, makes it that much more interesting.