U = Uniformity. Not to hasten to add of characters, readers should be able to tell them apart easily. What I mean here is if you have decided Character A is going to have a yellow bulging neck because that is how his species is made, then that should be shown uniformly throughout the story. Character A can't suddenly NOT have a yellow bulging neck by the end of the story. Whatever is vital to your character in terms of physical existence should be uniformly shown (and for the other characters who will be affected by the same things).
V = Variety. It had to be this really as it follows on from U above in every sense! Where variety does come in is via the traits your characters have. Not everyone is going to be meek and mild (just as well really, there'd be no story!). Not everyone is going to be hot tempered and causing fights all the time. The secret to a good story, of whatever length, is getting the balance right.
W = Writing that Flows. Every good story has this. Readers turn the pages, gripped by your characters, their world, and the situations you've put them in and your prose flows. The pace should be appropriate. The style of words used should be appropriate to the story and your intended audience. Nothing should jar the reader experience. And yes it is tough to achieve. It's never done on one edit!
X = Xeno. I've been looking forward to using this word again after having discovered it means strange! A really good story has to grip the reader. Something about your characters and situation should stir up their "HAVE to know what happens next" gene. Strangeness can do that in terms of strange characters, strange worlds etc. Sci-fi and fantasy depends on this. But even in an everyday setting, there should be something that draws your reader in. I've found this is generally down to an intriguing character that I've got to know more about. So what intrigues? There must be something "xeno" about them. For example, a fairy godmother who refuses to use magic. Now that's strange! Got to know more... you get the idea.
Y = You. I might be cheating a little here but I'll plough on anyway. What I mean here is you, as the writer, need to decide what tense you're writing your story in - fine, that's done, good. Okay next task is to make sure it is your characters' voices that your reader hears, not yours at all. From a reader's viewpoint, you here is to symbolize being totally engrossed with that story. The story should entice you in and keep you there until it is finished. So you and your reaction to a story is hugely important. A writer should be looking to make impact on the "you" they think will read their story.
And last but not least...
Z = Zest. A story essential, regardless of genre, is that it should be an enjoyable read. There should be a zest to it that gives the sense the writer loved writing it. (It does show). I've also found characters that have some zest to them are more lively and therefore better to read about than those without that quality.
Phew! Got there! Whatever you read and write, enjoy it. It's the single most important thing about the story. If you're the writer and you're not enjoying the process of writing it, why would anyone else enjoy reading it? If you're the reader and the story's not gripping you, put it aside for a while. Look at it again after a break. Does it still not grip you? Then read something else. Sometimes it can be a case of timing. I recall trying to read books when I was younger, not getting on with them, coming back to them later (sometimes via film adaptations) and loving them then.