The one thing I found most interesting was learning to be critical of your work; look at it for what it is, not what you want it to be. We should look at our art and ask ourselves, "If someone else had painted this, what would I think of it?"
I dunno about the rest of the world, but I definitely suffer from that. Occasionally to the point of being afraid to look for mistakes because I know I'll find some. So I thought that was a pretty sweet little tidbit, and definitely something to take hold of if you plan on improving your art.
And sorry I don't have an image today; all I did was some sketchbook doodles and don't have a scanner atm.
Hopefully these entries will get longer and more informative too, but for now I need to sleep.