Well, I only read at moderated archives (when I find time to read) because I don't want to slog through stories full of grammar and spelling errors. A few here and there I can live with because you see those even in published works. No one is perfect despite their best efforts. I do want my story have a well developed plot and generally I expect the major plot point to be set up in the first chapter. That's what will draw me in.

Obviously characterization is important as well. I need to recognize the characters from the story. Now if the fic is set in the future, I know that the characters will have changed, but I need enough information to understand how those changes came to be. I mean don't have Hermione as a rabid Quidditch fan unless you can really sell me on that. Also, don't have Snape suddenly metrosexual without a darned good explanation.

Now that's not to say the other choices wouldn't be a turn off for me, this is just the biggest turnoff. Not to say I don't like an occasional PWP, but not chapter after chapter of it.

Like one of the others, if characterization and/or plot is off, I generally won't make it past the first chapter. Rarely have I made it past the first chapter only to quit later, though there have been some stories that started out very good, but were drug on past their shelf life, probably because the author was getting a lot of reviews and decided to keep going.