I listened to a Zoom call with an art agent from Jackie Winters and some of the advice was very specific to an art agent's point of view. Some of the advice was also different to what I've heard you guys recommend, so it's really interesting to have that perspective too. It was very much a bussiness-y, prosaic way of looking at illustration, which was actually really stressing me out! 
But there was some very concrete advice that I don't know that an illustrator would know to give. If that makes sense 
@Mimi-Simon I'm going to push back a little bit on this advice (hope you don't mind! : ). The reasons you listed is exactly why illustrators SHOULD listen to agents. What you mentioned, emotion, is often the exact thing that is lacking from a lot of illustrators work. If you tell a good enough story and can bring in enough emotion from your viewer, all the art techniques don't actually matter that much. Most of the people who didn't make the top 16 in our contests could paint well enough, but the emotion was lacking.
You are right, agents don't care about what brush you used or how you did a certain thing in photoshop. They look at what you are actually saying with those things. And most of the time, us illustrators aren't saying enough. We are trying to impress other illustrators instead of being true to a narrative or emotion.