On the superficial side, I like blue and white combos, I like short hair, and I like energetic tomboys. Her voice is cute too (but Kyouko's is cuter).
Slightly less superficial, I adore the You Are The Monsters and tragic monsters plotlines and the fact she wasn't just magic'd back to life easily. I like how she managed to have her real life and magical life combine in such a way to muddle her up and drag her down, yet she matures by the very end. I feel a resonance with her happy-go-lucky to dark transformation as it reminds me of my struggles at her age, though in a more overt manner. She's an idiot, but she's also a child.
But this child has matured as of Rebellion. She is competent and capable, yet not horrifically overpowered. She's more forgiving and understanding - well, until Homura screws everything up.
She's cute, she's a screw-up, she learns.
I think the doujins are samey because they don't bother to explore what their relationship (and sexual life) could grow to be. And they make Kyouko too cutesy. She needs a bit of bite.