1. Are the characters stereotypical and predictable, or are the characters unique and surprising? You've seen them - the classic villain, the classic detective, the classic everything that makes the story far too predictable. The kind where you can guess who the guilty party is based on the first chapter alone. (This does not count the ones where you're allowed to watch the crime happen and step into the mind of the guilty one - those ones are very interesting!) No matter how far along in the mystery an author is, the reader can comment on the characterization of those participating in the story. Maybe fitting the stereotype was desired, and maybe it wasn't, but regardless of whether a story is finished, one can comment on the development of the characters.

2. Too much description, too little? In any genre, description is a key part. It can either halt a story completely dead just to describe a spec of dust in the room, or it can make a story flow naturally and make the reader feel as if they're taking a once-over the room with their own eyes.

3. Is it interesting? Even if you can't read more, do you want to? Would you bug the author, poke and prod them, to get another chapter just to satisfy some of the curiosities and questions? Or do you not mind at all that it's not finished and you're putting it aside because you've run out of parts to read?

4. How is the story progressing? If it is one where you get to step into the mind of the villain - are you still wondering how they will catch him, as he seems to keep slipping away? If it is one where you have to discover the guilty party - are you still guessing? Have they given you enough clues to eliminate some, but still keep wondering about others?

5. Are you enjoying it? Perhaps the most simplistic one, and the one that makes the most sense. Are you enjoying the read? It makes a large difference!