God that design is a fustercluck.
Anyway, the main thing I have against the design is that I think he's too dynamic. While badass, the pose is very active, which is very rarely the case with Gen V sprites. Put it next to just about any other sprite, including most legendaries, and you'll see what I mean.
That aside, if you did want to keep with this pose, the back third of the tail probably shouldn't be bending that much, as it breaks the momentum of the rest of the body. Maybe it would look better with shading, but currently, it looks like his front half is undulating vertically while the bottom half is undulating horizontally, which isn't something that serpents can pull off. Similarly, although to a lesser extent, the chains coming off of his jaws could also benefit from having either more of a consistent curvature with the rest of the sprite, as it currently looks like they start off trying to undulate only to end up going straight, which again breaks the momentum of the pose.
Again, I really feel that this pose is currently just too dynamic, and many of the issues stem from that.
Edit@Typhlito: tbh, I feel like this design could pulled off better than with a coil. Maybe drawing inspiration from the likes of Gyarados/Serperior wouldn't be a bad idea? If anything, I feel that it'd be better to break from the old coil pose if only to further differentiate the two, and because it'd likely look better uncoiled due to the silly head chains.