1. Strong, memorable characters.
2. An intriguing plot.
3. The story makes me laugh, or think, or react in some way. (That's how you know a story has had impact).
4. An ending that delivers on the promise of the opening lines.
5. Where there is a twist ending, for this to genuinely take me by surprise. I like to look back at a story and then spot the clues I missed first time around! (The great thing about doing that is you can learn so much from doing this and, of course, apply it to your own writing).
6. It is a story you are keen to read again and again and again. A Christmas Carol is a classic example of this for me.
7. It is a story you remember well. This doesn't stop you wanting to read it again because you will not recall all the details but you DO recall the pleasure this tale gave you and THAT is what you want to experience again.
8. You can easily envisage the story being a film. (This is a great test of how memorable the characters are and how strong the plot is).
9. It is a story that adds something to the language. Shakespeare takes top honours here.
10. It is a story that defines its genre or expands it. I'm thinking of Hans Christen Andersen here who added so many wonderful fairytales to that genre.