Hey man I did a very quick paintover I hope you don't mind.In my view, you did this piece thinking too much in 2D and this is the reason why you felt there is something wrong with it.If we find the horizon the boots don't follow the rule about ellipsis.Which is "The farther an ellipse goes from the horizon, the wider it gets".
There are some perspective problems in the armor and the legs.The face is 2D because it's in a complete profile and contradicts with the horizon.
Do to my personal taste :
I decidet to add fire for the sake of contrast, because the image was too dark.I also played with the light in order for the silhouette become more readable.In my opinion the detail in the knight draws to much attention in places where it is not needed so in the paintover I removed some of it.
I don't know perspective that well, but in my opinion the main problem in this piece is perspective.I apologize for my bad English :)
OFF : The left arm compared to the right is longer, it should be the other way around.