Characters that drive the plot, are the “authors of their own misfortune” or “create their own luck” as this means the characters have gripped the reader, are convincing and their powerful impact will make the story memorable, something every author wants.
A strong plot, a page turner, a story that you will remember years afterwards yet always be happy to re-read it.
I do like a decent one-liner but it must be appropriate to the character and follow out of the situation that person is in.
Sympathetic characters, characters with flaws who know they’ve got them and try to overcome them and I have a soft spot for villains who, while their actions are wrong, do have genuine reasons for acting the way they have (and that the good guys may not always be as good as you might think).
Missing the characters when the story’s finished. Or you can envisage how they would continue if the story went on further. This is a good test of how memorable characters are.
Words that don’t seem out of place, prose that flows and being able to tell the characters apart by how they speak.
A fast pace! No “boring” bits. I want to be gripped by the narrative as well as by the dialogue.