First of all, I really do love this. I love MLP:FiM and I love Discord. *Brohoof*
There aren't too many issues in this, but one thing did bother me. Discord is a draconequus, which means he has the head of a pony and then varied other parts. I think you made his head a little too dragonlike. I know that in the series, his head is really dragonlike, but it's still meant to be pony-ish. I'm not sure if I'm making sense.. Also, I think you forgot his fancy white eyebrows.
Other than some practice with painting and using a smaller brush to add details to fur and such, I don't have much to tell or else I'd be really nitpicking to the extreme. Well, perhaps you could try and add a furlike or scalelike texture to his torso, but I'm not sure whether it's meant to be the body or a snake or any another animal.. Discord's a rather odd creature. So, it's rather hard to actually help you with that.
I'm not sure if any of this was actually useful. Sorry if it wasn't. :c