It's A Wrap

I don't know your exact reasons, but I could tell why you put this on your scrapbook, it is certainly less detailed than other artworks of yours.

But first I will tell you what I liked from this piece. First, the cheer here is absolutely amazing and well portrayed, that's something you know how to do, to transmit an emotion trough an image. I like how Derpy is one of the only two ponies with opened eyes.
The background is simplistic but it doesn't needs to be to complex to look good (It looks good!)

I really like the shading and lightnings, they add a lot of spirit to the image, and is totally goodly placed.

The minor problems that I have with this piece are on the anatomy, I guess it is slightly incorrect because this is an old artwork so, instead of just saying that there are problems, let's see what the problems are.

The front legs of Rainbow Dash are a bit too short and they don't follow the principle of the show of starting the leg thinner in the elbow and then thicker in the knee and cannon and even wider in the heel (Although is the exact opposite in real life), here Rainbow's front legs are of the same wide from start to finish. The same happened with the front right leg of Firefly and the orange pony's both front legs and Derpy's Front left leg looks a bit short.

I don't see any problems with the back legs, the gaskins and hooks are well placed, at first I was going to point out that Rainbow's points of hips and stifles were a bit big and/or odd but I realized they weren't and it was just matter of the perspective that is used here.
I recommend you to take the drawings as if they almost were real horses with real anatomy, see how the legs and the body in general works and bends and based on that, you can start to play with poses that look natural on their anatomy but also look like a cartoon, not that you don't do it right already, just a tip to improve, improvement is always good, no matter how good you are.

All in all this is a very good, cheerful, colorful and full of good vibes deviation and of course the little oddities that I might find don't take it's essence away. You don't have to edit it and you don't have feel let down for making a tinny "mistake", you only have to have the critique and recommendations in mind so you make it better next time, rest assured that you will do it better.
Best of wishes on future pieces.

Your friend,
~Spark