I just downloaded the Left 4 Dead demo yesterday evening (containing the first two maps of the first chapter of the video game), and started to play yesterday late in the evening, and also today. I did not look for online coop gaming yet, because I first wanted to better understand how the game was working for me (it is possible to play alone with internet switched-off, in that case the computer plays the 3 other characters in the game).
I had some framerate problems with my PC yesterday, just after I downloaded the game (especially when I fired, or when a lot of zombies rushed at once). I thought it could be due to my (now) aging laptop, even if it still handles Episode 2 without any apparent glitches. But it was probably just because some of the PC resources were not released after the (1 hour) download and install, because when I played it today none of these problems re-appeared.
Me and my AI mates were able to make it through the first map on advanced settings (after many trials though), but we were not able to finish the second map successfully - yet ;-). We were very close the last time, but I was insufficiently prepared to the never ending wave of zombies when I pulled the switch (I think it's at the end of the map, just before the safe zone, but I'm not sure..). I heard the sobbing of a witch, I knew the dreadful thing was close, so I switched off the flashlight to avoid to awaken it. The problem was that there were a LOT of other zombies rushing at us, and the last wave was fatal: witch awakened, character dead, mission failed ;-)
As of the game, it is very efficient to scare the player. I'm sure it is much better when really playing it coop, but even in standalone mode it's scary !!! Hearing the witch soft sobbing is really something for example... And they handle shadows very efficiently making you never fully at ease anywhere in the game. Art direction seems flawless when you are in the game. They say this on the L4D blog, and I really thing it examplifies the way the game was designed: we'll talk about our approach to lighting the dark and murky world of Left 4 Dead, with a focus on playable, stylized dark — namely, finding a balance that gives a player the illusion of darkness without getting frustrating.
You never know from were the things will arrive, even if you backtrack, you may encounter new waves of zombies. It never seems like the so-called "A.I. Director" is spawning just them just in front of your eyes, I played the same parts several times (after many deaths), and although encounters of zombies were always different, they never seemed random. I think it is partly due to the fact that all the level ambiance is consistent with the way and places they will appear. I would very much like to hear more about the director, because it seems to be very efficient with its purpose...
All in all, I'm 90% sure I will buy this game !!!
Update: I made it to the end of the two first maps after all... But I think I saw a zombie vomiting on the floor once. Disgusting !!
Update 2: The zombies are not generic at all. Apart from the 5 bosses, there is a LOT of different zombie faces, not only say one for zombie men, and one for zombie women. And even if you don't really have the time to enjoy the view, the graphics are really good, very good. During my nth trip through the two levels, I took a lot of screenshots - while endangering the life or my character, and believe me: they are really gorgeous. And even in the height of action (brains, brains !!!), zombies are, sort of, say, beautiful. hmm only as zombies, I agree, zombie women are really not my type, first they seem to never wash their teeth after eating raw meat ;-)