There are two ways of approaching the creation of a card: modifying the gameplay, using all sorts of crazy turn manipulation or rule changes or whatever, or using whatever the card depicts for some motive, like humor or... art? This is not an either/or choice, however; any card can be placed on the sliding scale, as laid out by Professor Ickbweck:
Cards with just a picture and text, and absolutely no effect on the game whatsoever. As you may have surmised, this end of the scale can fall into the category of Useless Cards. But they can be pretty!
Point Cards fall under this category, as the points themselves are largely independent of whatever is on the card. Note that the drawing/flavor text on cards does not have to be good to fall under the Art categories.
What happens when the subject matter of the card and what it does are equally important to the card's merit. Generic Effect cards usually fall under this category, if the effect is compact and the picture has some effort put into it.
This is what happens when a quick doodle is scribbled to justify some long effect.
This is when the card is just a bit of text with some command or explaining some long-winded effect. Can be a result of smaller card sizes, as a complex effect would be crowded enough as is.