First of all: I used this image for comparison- [link]
Vision: I saw this on my DeviantWATCH and it really popped out! It stood out from the rest of the art for sure. It's just so big and colorful, and my brain lurves big and colorful . However, I wish you could've done a bit more detail in her mane and tail, maybe even a scene in the background. Maybe shade it, or add some shadow, or add a background behind her. I love shading and backgrounds, and that would help her stand out even more!
Originality: Well, I get that you're practicing with cel-shading, and you're just experimenting, but I wish you could've done something else in the piece, or make her in a slightly different pose. It's just...I've seen a pose like this before many times. Though I definitely think that this pose is hard, so I understand that this may be just as difficult as a more original pose.
Technique: I love the shading you did here! Good idea on keeping it minimal, since Celestia resembles sun, light, day. (On the other hand, Luna may need more shadows to stick to the "moon, dark, night" theme.) Also, on one note, I think you made the hooves a bit too wide. They look bigger than her head! It's sort of overtaking the picture a bit. Also maybe next time, you could make her neck a little less skinny. And as mentioned before, I think shading in the mane and tail, or adding a background, or even some textures, would make the piece stand out more.
Impact: The shading you tried here gives a huge impact on the whole thing! But again with the hooves; they seem to take over the picture here. My eyes keep moving to the hooves. But wow, this picture really blows me away! Keep practicing, and maybe work on some pony anatomy (it's hard to keep them proportional when they're sitting or lying down!), and you'll continue to impress me!
Thanks for the feedback. I can agree with pretty much everything you've said here: I do need to work on my backgrounds and Photoshop techniques, Even though this piece was merely for practice reasons I can agree that it seems a little empty.
Normally I would have added a shadow but, at this stage, I realised that her mane and tail would've caused the shadow to look off and, therefore, left it as was. I also wasn't entirely sure what to do with her mane and tail, to be honest, I thought shading it wouldn't look right because it does kind of glow but I can't get the effects right just yet, so I'm kinda writing her mane and tail off as a failure here. But I will keep experimenting.
I can also agree that her hooves are far too big and do sort of dominate the picture but the pose wasn't really meant to be original because I was just practicing. I will, however, admit that I missed out her other hind leg.