I guess I should have said "until I got done playing the demo" in my original post, I did spend more than five minutes with it.
There is something to be enjoyed in Left4Dead it's just not my cup of tea. Maybe if it didn't use the Source Engine or feel like a mod of HL2/Counterstrike/etc. I would have enjoyed it more, or maybe I was missing some grand gameplay element that the rest of you enjoyed.
I'm being overly negative. There were some things I liked. The environments were detailed and well constructed, the zombies acted like zombies should act, and the variety of zombies was great (seriously, do not startle the witch!). Given the choice between this and Resident Evil 5, I'd buy two copies of this and give the extra to the first unfortunate purchaser of RE5.
I'll give the demo a few more tries, but the game probably isn't going to go on my "must have" list. I will give it a novelty award for having the most idiotic non-acronym title. Left 4 Dead, 4 = for, four players. Hah! Oh Valve, what will you clever monkeys think of next?