There are lots of things to like from this piece. You have an amazing ability to handle line. No line is set without purpose; delicate, small, soft lines towards the background and firmer, bolder lines towards the foreground to develop a sense of depth. The tuning keys at the top are the best and, because you can see that they are tilted in different directions, it makes it obvious that you've paid a lot of attention to the small details that make the picture as a whole come to life.
There are a few things that I would point out to you about this work. The imp himself feels stiff in comparison to the fairies. With the fairies, they have all sorts of movement in their body shapes, like they're dancing or moving along with the music or swaying, at the very least. The imp is fairly straight. From the heel of his boot in the center of the picture up to the top of his hat is a very straight line. The stance gives him a center of gravity, but the shading lines and the pose lock him down in comparison to those surrounding him. It makes him a focal point, yet also restricts the dancing feel of the picture. My eyes, as the viewer, are drawn from the dancing fairies at the left hand side, up the neck of the instrument due to the angle of the strings, up to the tuning keys, and then straight down to the fairy with her mouthful on the right side. With all of the hard work you put in on the shading on your central character, my eyes are constantly compelled to travel away from him, possibly because he feels disconnected from the rest of the piece. He doesn't connect with the background mushrooms or really touch them like the grasshopper fairy and his lines don't flow as freely as the fairies on the left. I love staring at the top of the instrument though. For a suggestion, I would try to put a bit of rudimentary shading on the fairies themselves to bring them more towards the imps level of "finished" drawing; not the really intense shading that you've done that makes the face feel as if it has saggy, old skin, but something that shows just a little bit of depth (sort of like what you've done on the grasshopper), maybe even some small details on the fairy wings and drawing a bit more background in behind the imp to make him feel more part of the picture. The only other suggestion is to subdue the border lines on the grasshopper fairy. She has the darkest lines of the piece and draws attention because of those lines and her comic expression. Perhaps you should make her less of a focal piece by fading her out like the other fairies a bit more.
Overall, this really is a great piece to showcase your lovely detail abilities. The content is amusing, the lines are engaging, and it's a very solid piece all in all. It has a great sense of whimsy and fun about it and I really enjoy it. Keep up the nice work and thanks for sharing.
I can see your points and you are right, to tell the truth I've been a little lazy with this work because I just have had the inspiration to do it and realize it raw without thinking about focal points and all the other stuff that a serious illustrator should take care about so all you say is true and makes me think about use this as a sketch to realize a real illustration, with colors maybe, and taking care about all the things you pointed out, maybe I will find the time to do this someday