Turn-offs: Stories whose formatting makes it hard to read. This includes extremely long paragraphs, odd fonts, paragraphs that aren't spaced apart and poor background/foreground contrast. Please note that some of these apply online only -- they don't apply to a printed story.
What am I looking for? An early indication of a plot. I want something interesting to be happening. If I've read the first page and it appears like another "let me introduce myself" or "it's another day in the life of your average person" story, then I will often stop reading. I'm happy for the plot to develop slowly, but I need something early on to make the story interesting. A story where nothing much happens for the first five pages is unlikely to find me still reading by page six.
Apart from that major point, I want a story that I can read without having the mechanics being obvious. By that, I mean I'm not being distracted by trying to sort out what the author is trying to say. If I keep being jarred out of my involvement by issues with the writing itself, I'm not likely to continue.
After that, I just want it to seem believable. I'm pretty tolerant in what constitutes believable, but, as was once pointed out to me about an early draft of my own first story, teenagers don't talk like they have PhDs. My goal is to find myself 'inside' the story, not observing cynically from the outside. If what I read seems natural/realistic, then I'll keep reading. That applies to fantasy & science fiction, too. It is the characters, more than anything else, that makes a story feel real, and if I think they are acting the way I think they should act, then I'm happy. If they act in ways I don't expect and there doesn't appear to be a possible explanation, then I'm likely to stop reading.