I know what the answer needs to be if you want to be a successful illustrator. It comes down to this simple idea:
"You need to be able to effectively and creatively meet the needs of the client or job at hand." This is why the 3rd tuesday prompts are so good. It's easy to make cool illustrations if there is no criteria. It's hard to do it well while hitting a concept.
Things that reinforce that are:
Think carefully about what is needed in any given situation. You are able to adapt to the situation and still get a result that resonates
Experience and working in a team environment
Offering multiple choices to the client that all work
A willingness to work with others to get the best solution. A willingness to revise to get a good result.
Things that don' matter to a client (these are things that I hear a lot from students when they are entering the field):
I really LOVE to draw
I've been drawing since I was 2 years old.
You are willing to do whatever they want (they are looking for someone who comes up with solutions, not someone to dictate commands to).