You have got to admire Valve's ability to take a concept, simplify it to its essentials and then polish it to an exceptional degree. When it boils down to it there isn't all that much in Left 4 Dead but what is there is solid gold and crafted to perfection.
As expected single player mode is no more than an extended tutorial, worth playing through once to get used to the maps.
Everyone raves about versus mode where teams alternate between survivors and infected but I haven't gotten the hang of playing infected yet so for the moment I am playing mainly co-op, registered as mbp if anyone wants to look out for me on Steam.
MBPs tip - when things go completely haywire and you are over-run with an impossible number of infected thn get your back against a wall and melee melee melee.