V = Variation
Whatever genre you write in, there should be variation in the types of character you portray. A story with all heroes is no story at all! There has to be a villain somewhere (even if the characters concerned don't think they're villainous at all). Also, in a magical setting, there should be variation in how much magical ability characters have. If they all have unlimited powers, where is the conflict? You'd have a stalemate situation. The characters know they can't better each other, at least not with their powers. So vary things, mix things up, deliberately drop your characters in it, and see what happens! This is very much the fun side of writing. You will soon find out who your strong characters are.
W = World
The world you set your stories in can almost be a character in itself. It needs to be believable, no matter how fantastical its powers or setting. There must be things about that world we can identify with here. For example, every world has to be governed somehow so politics has to rear its (at times) very ugly head.
X = Xeno (meaning strange! Confession time: did have to look it up. I could have gone for X-rated for this but felt it was too obvious. I also want to remember this word next time I play Scrabble!).
So word of the week for me is this one then! Seriously, though, no matter how strange the world you've created is, there still has to be something about it that fascinates a reader. If it is too odd, you risk alienating the reader. Does that mean your world has to be a sensible one? Not necessarily. There has to be a point to what the world does. If it mines balloons for example, it would make sense to do that if is that world's chief export to its nearest neighbour. I did like Monsters Inc for showing why the monsters had to get the children's screams (they needed it as it was their power source) and I liked it even more when the monster world discovered making the children laugh was a greater source of power.
Y = You
The first fan of your writing should be you. Your fairytale, whoever it is aimed at, must first convince you. Is the magic a vital part of the story? It should be for a fairytale. Do your characters have to get themselves out of trouble without magic? Even better! You should enjoy what you write and at the same time be open to where it can be improved. Not an easy balance to strike which is why it is such a good idea to put work aside for a while and then re-read it. You will come back to the story with a more open mind, having had the break from it.
Z = Zippy
There will be many in the UK especially who will remember Zippy as a character from children's TV show, Rainbow! However for this, I mean zippy in terms of lively. Your fairytale should be a lively read from start to finish. Your characters should be lively and engage with your reader. (This is why if you enjoy your writing, there is every chance others will too. As for the feeling after reading something you've written "this is rubbish", bear in mind every writer goes through that. This is another reason for putting work away for a while before coming back to it).