The lighting is cool (backlighting! purple and blue highlights!), the details are cool (stars in River's skirt!), I love their postures and their expressions. And River's hair - oh the curls! The lovely sexy curls!
Since you say you don't like how it came out, would it help if I gave a bit of critique? (If not, ignore me!) I think the shadows on Eleven's face aren't quite right. I mean, he has a difficult face. Getting it right isn't easy. I think it's somewhere around the eyes that it's bothering me. Also, his hair seems too thin - it has a fair amount of body. For both of them, they seem to have more torso (waist to hip length) than they ought, but with the style of clothing they're wearing, it's hard to tell. Also, I think River ought to have her hand on the Doctor's lower arm, rather than his bicep.
Anyway, I only say these things because I think this is a beautiful picture as it is, and it has many details I like.
Awh this is cute. I love the doctor and that walk! River looks so beautiful and flowing and elegant and graceful and dsnjkn. The only fault I would have is the doctor's face. It looks a little bizarre? But apart from that, great pic.