Left 4 Dead Reborn is focused on bringing a more realistic and intense experience to Left 4 Dead. Currently version 1 is a simple config file, although it's changes shouldn't be dismissed easily. Only the most hardcore players will be able to survive a campaign on Advanced or Expert. Even on easier difficulties, this mod provides a greater challenge than standard Left 4 Dead.
A quick video of some gameplay on the Mercy Hospital campaign. I'm running L4DR V1, and playing on Expert difficulty.