AutoRun is the mechanism that proposes a default action when you insert an optical disc in the drive. For example, the contents of a file called AutoRun on the inserted CD is responsible for suggesting the action Run index.html.
Because of the rising incidence of malware that uses AutoRun to induce unwary users into running Trojan horses, the designers of Windows 7 decided to disable AutoRun capability on USB devices other than removable optical media.
Complete article: Understand why AutoRun no Longer Works for Some USB Devices
I was very happy this change was made.