One of the great things about playing on the PC is being able to extend a life of game via mods and hacks. It's one of the reasons why I usually pick up a PC version of a multi-platform game. Which is why I always enjoy things like this when it comes out and it makes me do a little experimenting.
If you own both Left 4 Dead
games like I do, there's a way to incorporate the maps from the first game into the second game. Details are here
but it takes a little elbow grease to get it working. But, if you want to enjoy the old campaigns with the new characters and zombies, here's your chance.