I just finished judging the magazine illustration categories for the Florida Magazine Association's 2008 Charlie Awards. I used their criteria for design, creativity, execution, etc., but also an important factor to me was how well the illustrations visually communicated the topic. That, after all, is the objective of magazine illustration.
My degree is in architecture, but a good part of my design school experience involved labs in visual communication. It's sometimes easy to get sidetracked from that objective when focusing too much on technique. Some of the entries I judged, while technically superior in rendering, did not communicate the essence of the article they were to illustrate.
I've found it helpful to allow a preliminary sketch to lie around for a day or so and then come back to it with a fresh eye to see if it conveys the message it's supposed to. A gag, particularly, can seem so logical to you when you're constructing it, because you know the visual punchline in advance. But sometimes a fresh look a day later can reveal that you've missed the proper set-up. What's really embarrassing is when you find out you not only missed the woods for the trees, but you weren't even in the forest!