flopsybunny:
I give a high score if I enjoy reading the story, but if it's sloppy writing or it has a heap of mistakes I deduct points ('Oh what a meanie!' I hear them cry :D), and I'll also award extra points for the quality of the writing (even if I don't like the story itself that much). If something stands out as being particularly well crafted/imaginative with a decent plot and believable characters and engages my interest, I may even give it a 10!
Yep, it's up to each reader to invent his/her own criteria, and it always works out very well in the end because there are so many people doing the scoring that the averages flatten things out.
My formula is very much like Flopsy's, except that a perfectly crafted story rates a maximum of an "8". So how do you get a "9" or a "10"? For that you need to trigger some emotion in this old Guy. Make me laugh (it happens), arouse me (yes, it's still possible), make me shed a tear (yep, that's still possible too), or even make your characters piss me off!
It also helps if after reading your story I wish that I had written it myself.