An illustration is like a movie: if there are too many special effects and explosions, the story seems to be less of the focus or sometimes even missing.
When you are creating an illustration, aim at the most important part of the storytelling, rather than shooting all over the place. Adding details and rendering everything to perfection is, for the most part, something you do out insecurity or lack of focus and control.
Tell the story, with as little sidetracks as possible, as little colour, elements and lines as you can. Use only deadly force when needed. Highlights are only for the important parts.
In illustration, it is all about the story.
I have this painting by Lucas Grasiano in my mind. The painting has so much story in it that I cannot help to put myself in the situation of the fleeing people. Notice how he did not try to arrange the figures in a heroic pose or made them look pretty. They are story pieces rather than decorations.
I always go back to this painting and try to make up version of a scene like this, in my mind, that I want to paint too...when I get the time. A painting that keeps living in the mind after your eyes have gone to look again on real life.
I have one illustration for magic that I think sums up most of what I think a good illustration needs.
It is a thief/assassin spying through the window of a ship. The colours are minimal, the use of warm light acts as a spotlight for the focal point and the level of details is muted down and kept within reason by the use of the simple palette. All lines point into the picture either towards the face or the area between the head and the window, to frame the character.
Harbor Bandit - Magic the Gathering
Just remember: If you can tone down, or even remove elements in your illustration, it lends so much more focus and power to the things that are left in.