The art of the classic comic book cover is dying. It may, in fact, already be dead.
This might be a case of me being old before my time, but I feel that a strong comic book cover should sell you on the events that are going to transpire within the book it is attached to. It should not be a generic image of a single character standing there looking cool. That is not a good cover - that is a pin-up, and it does nothing to tell you about the story except that the character on the cover might be in it, a fact which is itself not even guaranteed.
In this letter column piece from DC's Secret Origins #40, then-editor Mark Waid discusses what went into making a classic DC Comics cover, describing what elements Silver Age editors knew would entice kiddies to give up their dimes.
This is knowledge, I feel, that has become lost over the years. Maybe, with the state of comics being what they are, and the tendency to "write for the trade", covers have lost their importance as sales tools. However, I still feel that covers are an important part of the craft of comic-making, and that the knowledge in this article needs to be passed on - even if, ironically, I didn't think artist Bill Wray's cover for this issue was actually very good.
(That is probably not in fact irony.)