I'm playing the demo of this at the moment, along with Fallout 3. Fallout 3 is, of course, a much more immersive gaming experience than l4d will ever be. The upside of this, is that l4d is that quick zombie-massacring fix that everyone needs at one time or another.
The atmosphere of the game is great, with lots of ambient noise to freak you out when you're gaming at 2:00am with surround sound speakers (or decent headphones). The co-op portion is nice so far because you don't generally get TK-ing a-holes (though they do exist), and pretty much any of the specialized zombies you encounter will require the assistance of your teammates to bail you out and save your soon-to-be-eaten rear.
Overall, it isn't "how many shots does it take to kill a zombie," but "how many zombies can I kill before I run out of bullets and get eaten alive." Most things drop in one well-placed shot from a rifle or shotgun blast, though the sheer numbers are what get you in the end. Also, nothing like being completely disoriented and being eaten alive while you're lying on the ground hoping that your teammates aren't in the same position.
Most will never know what it is to be Chosen.
"...real life is a lot safer than Shadowbane; while my neighbors
upstairs are dickheads, they aren't in the habit of rounding up a few
dozen friends and besieging my apartment, battering my furniture with
hammers, and setting a bane circle on my ficus tree."