Valve says that the new weapon progression introduces greater diversity, and this is seemingly evident on the full-auto route, as you move up from silenced Mac-10 (or perhaps TMP?) past AK-47 to the biggest assault rifle. The most obvious and striking departure on the weapon front, however, is the grenade launcher - a one-shot-at-a-time friendly-fire nightmare that makes the player a lot more dangerous to both sides. You can destroy a horde of zombies in one blow, but you have to switch to a side arm or think twice about trying to dislodge a Hunter from an ally, unless you want to loose them both the mortal coil. It's such a game-changer that it effectively represents a profound change of player class in a game that only does ad hoc classing.
It's enormously useful when I uncover it in the second campaign we try out, Swamp Fever, which is also the most exotic of the three shown so far. Almost everything about the two Swamp Fever chapters we play is brilliant - the mud men uncommon common biting at your knees and splattering mud in your eyes to announce their presence in the waist-deep swamp water; the plank pathways in the mud and raised walkways; the downed aeroplane with one wing sunk into the water and a massive rip through the fuselage that you have to scramble through; and the best crescendo moment so far, on the banks of a river waiting for a rusty old ferry platform to be hauled across to your side of the water.
The level configuration is cunning and, despite the setting, fresh, driving you into new patterns of play. Infected rise out of the water, the cloying dankness of which slows you down terribly whenever you need to wade into it, and there are some lovely touches here, closer to the marginal flourishes in Portal or Episode Two, like an assault rifle upgrade to be pulled from the cold dead hands of a CEDA man dangling by a parachute in the trees, and all sorts of junk you wouldn't expect to see in a swamp. Faliszek explains that this sort of stuff really is there in real life - a veteran of New Orleans, he even sent unbelieving researchers down to the bayou to see for themselves.
There's much more to talk about - so much more, in fact, that I brought Christian Donlan along with me, a man as horribly polite as the new Jockey infected is horribly effective. You should check out his alternative hands-on to find out about the special infected updates, melee weapons, and the manner in which Left 4 Dead's visual identity is beginning to take shape.
The three main things we don't get to play around with this time, apart from the remaining two campaigns of course, are new finales, the special infected side of Versus mode, and the promised new game mode, which has yet to be announced. Faliszek says the upcoming Crash Course DLC for Left 4 Dead 1 will introduce a few of the key special infected interface changes, like allowing you to see recharge times for your team-mates' attacks, and the one finale we have played - the gauntlet conclusion to The Parish - at least bodes well. Fantastically well, actually. All questions about the new game mode are beaten back with a frying pan.
But that's okay, because the last word should go to the AI director. He's still in training, no doubt practicing his sudden, visibility-slashing rainstorms and working out where to drop the dynamic crypts in one of the graveyard sequences. But he's everywhere throughout our two hours, dumping a Witch on the wing of the swamp-sunk plane-wreck, lamping me with a Charger when I'm the grenade-launching survivor of a group with three men down and what I really need is room to manoeuvre, and laughing us out of the building with a Tank at the last of gauntlet Parish run, just as we haul ourselves off the bridge and make for safety.
He's older, he's bolder, and he still deserves his star billing. Unless he goes a bit Lindsay Lohan, it's hard to imagine Left 4 Dead 2 being anything other than magnificent.
Left 4 Dead 2 is due out for PC on 17th November via Steam, and at European retail for PC and Xbox 360 on 20th November.