Now i'd like to make this clear from the start. Im no artist. (just check out my gallery for that ) But I'd like to attempt at critiquing this piece of work, using your very own guidance/rant on your much-loved Tumblr mod blog page.
As a start point, I can see the direction of the light very clearly thanks to the shading, which is solid and shows good continuity. (something I have struggled with) Also, the direction of movement is nicely created with the forces amplified well using manes and tails of each character and also the slight arc bending from the rear of the carriage to the tip of Rainbow Dash's hoof. Although subtle, it adds a nice weight to the image and adds to the direction of movement, having it not only in a straight line. Another positive point is the work on using, what I will describe as "woosh lines", at such angles that give the image more depth in conjunction with the shrinking of the image from its perspective. Its again subtle, but marks as a great touch. As for negative points, I honestly cannot see many. One nit-pick id say is that the background looks slightly out of place, where the clouds look 'less cartoon-like" than the characters. Although I am fairly sure this is just artistic preference. Another small point is the lack of shading for where the axis of the wheels connect to the carriage, I feel that these gaps between carriage and wheel should be darkened more as their positioning is very cut off from the light. I could be wrong, but this is just my point of view.
That is just about all I have to say about this piece. Overall I really like how it has turned out but these small features I think could be given some attention to really make objects on the page stand out.
Thank you for your time and I hope I have helped you in some way