I voted for "Style, Tone, and Language", though I was sorely tempted to vote either for Characterization or Plot -- it's too bad you can't vote for more than one.
The reason I voted for the above is that I think, above all, a writer of fiction needs to be able to use words effectively and poetically, whether it be writing dialogue or description, both of which help to characterize the characters. And a plot is only the idea -- it takes a good use of language in order to translate it into a well-crafted story.
Ultimately, all or most of the elements in the poll need to be taken into consideration when writing or critiquing fiction.
