I don't know if its that there's nothing to the characters or nothing to their stories, but I have dropped both these books out of disinterest.

Can someone direct me to some good Nightwing or Batgirl stories, if they exist?

Or, can someone explain these characters to me, and why people love them so much?

Batgirl has left no impression me, but this is what I like about Nightwing:
-He was Robin
-His relationship with Bruce
-'Coming of age' stuff; stepping out of someone's shadow, etc
-His personality

But what makes Nightwing interesting now that he's grown up? What does he believe in? What are his faults?

Ditto for Batgirl; what does she believe in? What are her faults? What does she have to deal with that's unique/interesting?