When I heard about Left 4 Dead, I was really expecting more. I hadn't seen a trailer, or anything, and that was it. People just told me about it.
I was expecting NPCs to interact with, rather than a map to run through. I thought it would have more variety. I thought the game would be like a full on adventure available with co-op where instead of a map you run through with panic events, you interact with other survivor NPCs, be asked to go and rescue a survivor's friend from a building full of zombies, defend a house with a few other survivors, and a bunch of other stuff. What it is now is just maps and panic events.
Not that I'm saying L4D is bad, it's great, but I think it would've been cooler and maybe less repetitive if it was something like this. It just seemed like they could do so much more.
And what I saw from the trailer looked pretty cool, too, I would've loved having to sprint the bridge while it was being blown apart, meaning you had to get across otherwise you'd really die, but they ended up changing that.