For newcomers: L4D is an online first-person shooter in which you and three others team up against hordes of zombies (which can include four opposition players). A computer-controlled 'Director' dynamically alters the difficulty level depending on how you're doing. Do not buy L4D or the sequel if you only wish to play single-player.
The rest of this review is chiefly for the purpose of informing those who've played the original L4D.
First things first, Left 4 Dead 2 has not come too soon. This is a proper sequel, not an expansion. For one thing, we get five all-new campaigns. On top of this are the cosmetic improvements, such as the slightly improved graphics, and the ability to lop off individual zombie limbs. There are subtle changes to the game dynamics, like the inclusion of more, smaller horde rushes. And then there are the big changes to the game structure, as when the Director changes the scenery to force you through a different route.
Those who played the demo (from The Parish campaign) will have enjoyed the chase to the alarm. These chase sections are repeated numerous times, most memorably during the Dark Carnival campaign, where the team must dash along a rollercoaster to hit a switch, whilst common infected swarm and smokers take pot shots from the gantries.
The final 'boss' battles are, in some cases, slightly more complex than the original. For instance, at the end of Dead Center, the team must gather a certain amount of fuel for their escape vehicle, negating the oft-used camp-and-defend option. I won't spoil the conclusion to The Parish, the last campaign - it should simply be experienced. You'll also be excited/terrified to know that the finales now throw tag-teaming tanks at you from time to time.
Each campaign now has its own semi-special infected. For instance, the labourers in Hard Rain can't hear your pipe bombs because of their ear-guards, while honk-nosed clowns in Dark Carnival lead groups of infected like undead pied pipers.
The new weapons are fantastic. Headshotters will take particular glee in the new 30-round clip sniper rifle, whilst the more maniacal will find heaven in the barrel of the devastating grenade launcher. There are also new pickups in the form of the bile bomb, which attracts the horde for 30-odd seconds, and the adrenaline shot, which does what it says on the syringe.
The only slightly unwelcome new inclusions, in my opinion, are the weapon upgrades: incendiary and explosive rounds, and laser sights. For me, these additions detracted slightly from the atmospheric, survivalist experience when they appeared as mods in the original game, so I'd rather they'd been left out of the sequel.
Other than that, it's an essential purchase, especially the superior, cheaper PC version.