I recently discovered a large series of articles
concerning artistic theory, by the author of Dresden Codak. Most all of these are worth looking at and discussing, and they come with helpful illustrations throughout that show good or bad art and indicate why it is good or bad. Given the breadth of these articles, a particular one
is being used for the opening post, though one thread should be enough for the lot of them. Some of the key conceptual illustrations are below, though these are more to pique interest than anything else.
Note that these images are very rough sketches, and not indicative of the comic as a whole. As far as discussion goes, I consider most of the analysis brilliant, and immensely helpful to artists.