my vote was:
quality of Writing
quality of art
characters
marketing and promotion.
I think of it more as a "math equation" involving multiplication:
writing X art X characters X marketing= level of success.
If any of these is 0, your level of success will be 0. If you improve one of your weaker spots, you may get some exponential growth, and sometimes one or 2 of these weaker factors may hinder you, despite having an insanely high level in one of these points. Also you gotta know how to capitalize on your strenghts.