Really nice work IMO, you'v gotten the camera match and lighting down almost perfectly and it's an interesting theme... makes you want to see what will happen next.

Now as for crits (and this is just my 2c)

1. I'm having some trouble judging the scale of the dragon and how 'deep' it is in the scene. ie is it a huge dragon way above in the trees or a small dragon above the girl's head or where. The main reason for this is that it is un-naturally crisp and has no depth of field on it. You should blur it to match the depth of its placement in the scene. eg if it is near the trees then it should be as blurry as them. Same thing for the guy. Also since the dragon is presumably moving, it should have motion blur as well, especially on its wings. Otherwise it looks like a stationary prop someone hung in the sky. I would also change its color a bit so it becomes less lost in the green of the trees.

2. To me the horse seems to be floating on the path. The main reason for this is it's hooves have no contact shadows. Look at the rocks on the left of the path, notice how they have well defined shadow/dark areas at their bases ? You need to duplicate that for the hooves as well to ground the horse. The soft diffuse shadow works fine for the bulk of the horse but not the hooves cause even on very cloudy days, things that are almost touching each other get strong contact shadows. Easiest would be to just paint them in. Same thing for the guy and his boots. Just keep em subtle.

3. The grain and quality of the 3d elements is different from the backdrop. This is why for best results you should never ever use jpg files as backdrops. Most jpgs you get on the net have compression artifacts (those blocky things around edges) while your render doesnt. So either use a clean uncompressed file like a BMP or TIFF or first compress your render separately and then composite it over the backdrop so that both have artifacts.

Besides that it's excellent work !