Achievements were a major factor for this games replayability.
In the first game you had stuff like campaign with only pistols, no friendly fire, no healthpacks, ... on the second game there were a lot less, of these that required a full campaign run through with different objectives. The mutators kinda made up for that, but with the new DLC there was no such thing. Left4Dead has no story whatsoever and not real gameplay variety over the course of the game, so what is it driving you through the game? Achievements are one of the major points. Even though I still would've enjoyed the game without achievements I think I would have played it much less, especially the first one and I would say, that achievements are more important on this game than any other, so if they don't give us achs, they should include unlockables. Maybe have like 20 ingame challenges and after each 5 unlock one of the original campaigns.
However doesn't matter anymore since they decided to go with none. Still way better value than most of the DLCs out there.