Oh I'm so glad someone else is too 'territorial' to collaborate on a fic too...
More contextually, I'd say it's my ability to develop a complex story and story-tell it, and make the characters real and believable.
Absolutely; though I agree with the previous comment that it's an understatement. I love the complexity of your stories, I love knowing that I'll have to think about what I'm reading in order to pick up on everything.