UGO writes: "The melee weapons. The addition of guitars, axes, chainsaws and other objects make Left4Dead 2 more about hands on combat than ever before. There's something sickly satisfying about lopping off zombie heads with an electric guitar - Frank West beware.
The winding levels. It seemed that there were more dead ends, curves and winding paths than in Left4Dead. I found myself making some poor navigation decisions a few times, which made each turn I made that much more precious and potentially life threatening. The first game of the series was more or less straight paths with a few options to get lost, but Valve seems to have taken unpredictability up a few notches."