Why isn't the Metal Slug series as well-known as Contra in the United States? Sure, Contra came first but Metal Slug took that basic gameplay concept and threw in wonderfully animated sprite-based graphics as well as a sense of humor.
Perhaps it's simply a case of diminishing returns. 2007's Contra 4 on the Nintendo DS was hard as hell, but the overall experience was exactly what its fans were hoping for. By contrast, Metal Slug 7 on DS managed to screw with the formula by abandoning cooperative play. The graphics were ill-served by the downgrade to the small screen, too.
Metal Slug XX (pronounced "double ex"), a PSP version of Metal Slug 7, looks to correct those mistakes, starting with the return of multiplayer mode.