This is a very very nice picture of Derpy! She just looks so happy and cheerful in this picture!
I absolutely love the colors you used here. Using warm, happy yellows really makes the piece have that sunny look about it. The lens flare is also a nice touch as well. For the most part, you handled the lighting well on Derpy.
My biggest concern about this picture is that bent leg in the front there. It just doesn't seem long enough, because I don't think there's enough of the upper part of the leg suggested there. Whatever it is, the leg seems short in comparison to the rest of the legs.
The finished work looks much better than the sketch. It looks really good as for 4 hours; gosh, I spend like 15-30 hours or even more on mine too .
I like this drawing. Her pose is great (just like the overall flow of her body - it's very dynamic) and her mane looks very good. The "sunset" background and the lights on pony are well-done - it really looks like a sunset effect.
Still, even if I like this work, when I look at it I feel something is amiss. After a while I figured out what: Tongues are usually lighter than mouths. Try to invert those colours next time. Her eyes seems to be too big and I've got a feeling they are about to drop off her head (maybe it's just me?). I also think her angelic wings don't go well with her - those feathers are very think and the wings look too fragile to hold an adult pony.
And I guess that's everything. It really looks good and much better than your previous recent mlp works.
Also, there is a troll at the bottom of this page, MochaPK - he really can't draw even quarter good as you and he's jealous. Well, I think oblivion isn't a good place for you at all. Such comments with no explanation why somebody doesn't like one's work are pointless.