Hmm. I am not quite sure what is the most important to me, but if I had to choose, it would be a combination of good grammar, good spelling and good writing. Now, good writing does not mean having great or unconventional ideas, it is merely putting one's thoughts into the proper format. Even the greatest of ideas can be ruined by sloppy pacing, unlikeable characters or a sheer amount of lack of properly used literary devices. A plot idea can be grand in scale, awesome and awe-inspiring, but if it hidden behind ridiculously short or long sentences, bad dialogue or just bad presentation, it will come across as a failure. In contrast, with the proper use of literary devices, even the most clichéd and repetative stories can be turned into epic conflicts spanning decades.