I realise it's a game intended for continuous multiplayer replay value, but the lack of story does detract a bit for some people. Especially considering this is valve we are talking about, it seems a bit of a waste of a potentially amazing oppurtunity.
A lot of people have said, pong (and other similar games) doesn't have a story line but there is no human aspect to those games. Left 4 dead has 4 distinct characters with names, faces and personalities and the lack of backstory and character development is a dissapointment. Don't get me wrong I love left 4 dead but you can't expect to create characters and have everyone be happy to have so little explanation as to what's going on.
But I get that after the fourteenth bajillion time through the same cutscenes it could get completely agonizingly boring. The idea of a story played out in cutscenes between campaigns is infeasible but there's no reason why they can't have cutscenes exclusive to the single player campaign.
Alternatively why not have an option to create your own character. I mean plenty of games do it these days, all it would take is 20 minutes to make a face and what not and off you go. Because as much as everyone says "make up your own story" if I was going to make up my own story, the characters would be from space, from an underground civilization or similarly bizarre locales and backgrounds. So I mean if you don't want stories don't have such individual characters, but if you want the characters at least give some indication of why we should care enough about them to not let the zombies chow down.