Things to Consider...
Simply put, the overall quality of the art itself. We strive for quality over quantity.
Quite often designs are turned down on the principal that nearly identical designs have already been printed. A simple Internet search can ensure you don't design something that already exists. New takes on old designs are always welcome.
Careful attention to the brands/design styles used is critical. What's hot, what's not? Your designs content is key to success.
Timing is everything. Did an Internet meme pop-up overnight? Has something happened in the media that's worth designing around? Always strive to be the first to market!
There are great designs, and then there are designs that just have that "IT" factor. Nobody can really put his or her finger on it, but when you've got it you know it!