"To outsiders, the changes in Modern Warfare 2 might sound minor. But if you've played a significant amount of Call of Duty 4, they range from "neat idea" to "totally mindblowing."
Modern Warfare 2 represents Infinity Ward's first work since Call of Duty 4: Modern Warfare raised the developer's profile (not to mention its wallet) several orders of magnitude. But rather than taking a ton of huge big-budget risks, Modern Warfare 2's multiplayer carefully builds upon the developer's previous success in ways that will astound people who have spent the last two years learning every corner of every map in the previous game. Instead, the risky stuff is saved for the single-player campaign, which offers tightly packed thrills and rock-solid gameplay, though the narrative so wild it can get exhausting...."