In my opinion, the covers that really stand out to me are the ones that either highlight an important and pivotal part of the story, or shows some sort of threat to the heroes. The villain doesn't have to be shown necessarily, but so long as there is an inferred villain, that adds to the intrigue. Looking at this cover, you don't see the villain (or villains), but you can see that the heroes are ready to put up a fight and you can also see a bit of fear. That conveys the importance of what is going to occur in this issue and it draws the reader to it. The most important part of the cover is the intrigue that it awakens in the reader. If there is no intrigue, then it would most likely fail as a cover. Each book and every reader is different though, so if something works well for a specific audience, then I would say to go with what has been shown to work best.
