No Mercy and the Survivors being ported to L4D2 is great for a 360 player like myself.Valve is creating a new add-on campaign called The Sacrifice. The campaign is being released for both Left 4 Dead 1 and Left 4 Dead 2. Whether or not the actual maps will be identical or not is unknown at the moment.
The Sacrifice in Left 4 Dead 1 will feature the new campaign, but none of the features of Left 4 Dead 2. Melee weapons, new guns, new infected, etc. will NOT be added to the L4D1 version of the campaign. It's simply a new set of maps to play through.
The Left 4 Dead 2 version will still feature the same Left 4 Dead 1 survivors (Bill, Zoey, Francis, Louis), but will have all the features of Left 4 Dead 2 as well, including the new weapons and infected. It will basically be like a new set of maps for Left 4 Dead 2, as well as the L4D1 survivors being ported.
In both versions of The Sacrifice, players will be able to choose which of the four survivors sacrifices him/herself at the end of the campaign. [You all know who the canon choice is anyway.]
In addition, the No Mercy campaign is also being ported to Left 4 Dead 2. L4D2's No Mercy will have the original Left 4 Dead 1 survivors, as well as all the new weapons and new infected of L4D2.
Valve is also making a 150-page digital comic to be released with The Sacrifice giving background on the L4D1 survivor's lives before and during the infection, as well as the final canon sacrifice by [that survivor] before The Passing.
It comes out Fall-ish in Valve Time.
Also, this may be related to the lines found in The Passing with the original survivors talking about some boat and other non-Passing objects. That would also mean this is not the last DLC for L4D2, as other lines were found of the new survivors talking about snow.