This is a really good piece. I like the way you did the shading and light. The background is beautiful, the shading in the sky is very detailed and I love that, especially the streaks of light going through the clouds. I would work on the eyes, though. They aren't very clear and blurred just a bit too much. Some of the shading is a bit off, like the shading on the body of Twilight. The hair is a little odd and "choppy" like you just sheared it off. The background draws my eyes more than the "centerpiece" of the drawing, Twilight. She just isn"t all that "prominent". Other than those minute things I love this!! Good picture!! ^^
I am not a professional, so perhaps my opinion isn't as well informed as others. I really like this piece, and if you want me to let you know what I think would improve it, I'll be happy to throw in my two bits.
I really like the Twilight Sparkle looking at the sparkling twilight theme. It's clever and her eyes are obviously looking at the last sparking star in the sky. And... ho ho, she only has four stars on her cutie mark (at least that I can see)! It makes me wonder if that is a wistful smile, perhaps one of her friends has departed and she is recalling fond memories.
I agree with Xenaleena about the definition of the eyes. The eyes of the ponies are able to express an incredible range of emotions because they are large, colorful and immediately draw the eye. I like the shape and color, but they need to look more liquid.
Where is the light coming from? It looks properly pespectived in the background (which I love) but her face really shouldn't be illuminated, unless she was casting a spell and her horn was glowing. If we assume there is a light source behind the viewer (a torch?), there should be more light at the bottom of the picture in front of her and reflections off the water. If her face was in the dark, her eyes could still be luminous. Also her outline would be brighter than the shadowed portion of her body.
Also, I think there should be more splashing in that water to convey movement. A horse moving through shallow water is an opportunity to express power and motion. She looks like she is paused mid-stride. Perhaps this is intentional as she stops to remember a friend.
As regards the grass, I think it would be better if the tips of the grass were capturing the morning light across more of the surface. I notice the effect on the left side of the light spot, and I think that could be replicated along the top of most of the dark grass surface all the way to the left of the picture. Think of the contour and how the light would stop at a crest but continue to pick out highlights behind it.
I think the transition from stream to lake could look more natural. There would be different plants along the lake shore and the stream would widen at the mouth.
For the mane and tail, that is the one thing I actually don't care for, sorry. You should have the highlights follow the line of the texture lines, not cross them. It makes it seem like the highlights have been spraypainted on and are not part of her hair.
As a fellow deviant, I understand that after a while you reach a point of diminishing returns and you just want to get your work out there. Don't take my criticisms too hard, I think this was very well done indeed, but your vision exceeded your execution just a bit. Me, I do cartoons and jokes, so my work is rather less challenging. I'm talking about doing stuff that would probably add hours and hours to your project, but you obviously have talent and love the work, so perhaps you could invest that time. I apologize if I have been nitpicking.