as a reader what catches my eyes is the plot: how deep are the character involved in it, how credible are them and what is happening around them and to them, how I got involved to the point of waiting for a new chapter -or to finish a book-.
The characterization is also important: the feelings involved, how the emotions are shown and depicted.
Sex, for me, is a big turn off: I usually skip the graphic descriptions and well, it is someway boring reading in each and every chapter the same stuff happening :_)