This is just a guess, but I would say that it will be the same as orange box or perhaps a little more... mainly because in all the footage I have seen there are dynamic shadows and light sources, motion blur and perhaps even depth of field... There also seems to be more advanced physics on the character models... Big blobby dude's fat jiggles... Big huge zombie dude's muscles move... Lots of detailed, individual sprites on screen at the same time...
I'm sure there will be an option to turn details down, but a safe bet will be if you run Orange Box, L4D works just fine.
I found these system requirements.. at the end of the description. Do these look right? Hope not.. cause I don't have a 2.4 GHz processor! http://www.amazon.com/Electronic-Arts-Left-4-Dead/dp/B000PS4X7S
There are many zombies onscreen at the same time, and more than 1 dynamic light source. In ep2 it was only your flashlight I think, but in Left 4 Dead, if the 4 players turn on their flashlights at the same time it gets very demanding (for a source game). Of course you can turn down the details, I don't think you should worry about that unless you have a very old PC. The Source Engine has always been very nice to everyone.
I wouldn't say that just yet, everything is guess work and speculation until an official announcement.
They don't? Really? Oh well it would be so nice. The flashlights shadows from the other players would keep distracting me while the infected attack over and over and over again... would be so nice...
Maybe this will be like in TF2 where Valve focused on the gameplay instead of showing off the engine. I mean that's what hl2 episodes are for, right?