Survivors must scour specially designed maps for gas canisters, which they must return to a safe house to refill a generator. As the generator is filled, the Survivors earn points and more time is added. Meanwhile, the boss Infected -- including the new Spitter, Jockey, and Charger -- must prevent the Survivors from refueling by attacking them, destroying the gas cans, or running out the clock.
There are also reports of a "Realism" mode, but Valve has not officially announced details. According to the folks over at 1UP, Valve's Chet Faliszek commented that the mode is a "special modifier" that they forgot to hide for the demo. Doug Lombardi, another Valve employee, said Realism mode was for the "super hardcore player."
In related news, Left 4 Dead 2 has finally received an MA15+ classification in Australia -- though, it's a watered down version. The game "no longer contains depictions of decapitations, dismemberment, wound detail or piles of bodies lying about the environment." Valve is still hoping to obtain approval for an unedited version of the title.