well, played the demo (PC)
it is running a bit less fluid that L4D to me (perhaps a light technical upgrade of the game making my "average" PC suffer)
it is quite similar to L4D
it is not anymore dark (at least this campaign) but it is not a problem
I stil dont know very well hte new infected but they seem quite different
the TANK has a LOT OF HP
lot of new weapon
conclusion : if you like L4D, you should like it
personnal conclusion : I like L4D but I m not that much in this game anymore
it is like after some time it is getting boring that is why I doubt I will get the "2" ...
EDIT : I m gonna play a bit more to see others points