The original Left 4 Dead worked with the conceit that you were actually a character in a movie about a zombie apocalypse. The characters from that game were cardboard cut-outs, the stuff of slasher fiction: a grizzled veteran without a war to fight, an energetic yuppie who has been ripped from his life of urban familiarity, a dimwitted thug with a heart of gold, and a hot chick. The quartet fights their way through a city that has already fallen to pieces. The tone of the game was highly stylized, but the panic events and finales had a certain realism to them.
If Left 4 Dead was a slasher, Left 4 Dead 2 is a grindhouse flick. Valve has turned everything up: larger maps, more zombies, more outrageous events in a bleaker and crazier setting. The second campaign of the game takes place in an amusement park where you're chased around by zombie clowns. It ends when you take the stage that had been set up for the band Midnight Riders and set off pyrotechnics to attract a passing helicopter, all while beating zombies to hell with the band's guitars. Bye-bye realism.
For example, in the first campaign, an NPC (non-playable character, for those of you playing along at home) agrees to clear a path for you, which he does by setting off an explosion. He does this under the condition that you go across the street to the store and get him some cola--which will set off an alarm and summon the horde. Before you go, he lets you raid his gun shop, providing a nice kid-in-a-candy store moment. None of this makes any sense if you look at them through a lens of reality, but it works.
In part, Valve gets away with this by giving us more grounded characters. Our heroes have a bit more personality and depth than their predecessors, and there's a level of storytelling layered into the campaigns that deepens the game without interrupting it, all using character dialog and scenery. In many ways, the storytelling elements of this game are dramatically influenced by Portal--with setting and character interactions driving the story rather than plot. Not surprisingly, the game is pretty immersing: the levels are still linear, but they don't feel linear. You're still being led around by the nose, but you don't feel like you are. You feel lost without actually being lost. The background plot is there if you want to find it, but the real story comes from the characters.
In short, Left 4 Dead 2 is what a sequel is supposed to be: a better game. It builds on all the strengths of the original and improves upon the weaknesses. And through all of this, it never takes itself too seriously. There's kind of a push these days to make games "cinematic", but these games often fall pretty flat. It's amusing to me that Left 4 Dead and it's sequel are games that pretend to be cinematic, but focus on being quality gaming experiences. In fact, if they ever made a movie of the campaigns in the L4D franchise, it'd probably be a pretty horrible movie, but it'd be a movie that looked like it had been blast to make.