What is the story trying to get across to you as a reader?
Do you get a good view of who the characters are and what their personalities are like? Are there any characters that you have too much information on because they should be more minor characters, or characters that you'd like to have a better view of?
Does the plot flow logically? The story may have flashbacks, looking back at the past, but are these used at logical times? Are they helpful to you? Does the story's ending make sense? Does it get going right away or does it seem to drag?
Does the story use dialog appropriately? Can you hear the characters talking, or is the dialog fake or forced sounding? Are there places where characters could be speaking? Is the dialog easy to read? Where is it not?
Is the setting/place of the story described well, or is it unimportant? If it is important, are there enough details? Are there so many they are distracting?
Overall, what did you like and not like about the story?
Keep in mind that there are different types of fiction with different audiences, like sci-fi, historical, romance, etc. Even if a writer's genre isn't your own, what would make you want to read the story? Writers are always looking for new readers.