The first thing I'd like to comment on is the ingenuity of Cloudsdale. not only have you captured the magnificence of such a city, but you created it from your own perspective and given it a breath of life. It looks less like a floating city and more like a kingdom in the sky. The shading gives it a feel that sets it apart from the rest of the sky but makes it fit in just perfectly. The sense of perspective is really made whole by Rainbow Dash, who also looks expertly drawn, giving off a sense of pastel realism. The attention to detail in the background is astonishing, considering the piece is meant to attract attention to the pony and city. the clouds, the faint rainbows beneath Cloudsdale, and the landscape give it a full sense of completeness that, without the perfect touches you've given it, would keep this picture seeming to have that 'something that'snot quite right' about it. Every aspect contributes, rather than distracts or overpowers the other parts. I'm truly amazed at the quality of this, and how much effort must have been put into the development of something like this.
Personally I would have given myself lower on the Technique scale I didn't spend as much time as you think I did on the background other than Cloudsdale, with it being the focal point. I thought the focal point was stretched between Cloudsdale and Rainbow Dash, which could be improved upon.
But thank you!