Potentially great, and potentially bad... its all potential.
It is potentially the best multiplayer game you and friends could play available at the moment, providing tons of opportunities for fun, hilarious and fear filled game session, an absolute blast in a Lan party.... its also potentially a nightmare of grieving, lag filled torment and outright frustration when the servers are bad, and the players on the teams are not working together for fun.
L4D simply relies too much on players to make the game work, and as it has a limited number of slots for players to join...even one bad player can ruin a session. L4D does have some mechanisms to counter the bad, such as vote kicks and ability to tie your own dedicated servers to your prefered steam groups.
The graphics in L4D are simple, they do a good job of showing you the world and you dont need the latest technology to see them, the graphics however wont amaze you and isnt suitable for showing off your new graphics card.
The audio in the game is actually very good and adds to the athmosphere, I doubt very much players will be turning the audio off so they can listen to thier mp3 collections.
L4D ships with 4 Scenerios ( can also call them 'movies') that each have 5 maps, and two modes of gameplay... Campaign or Versus.
In Campaign mode a team of 4 Survivors must make thier way through 5 maps of one of the 4 Scenerios whilst under constant threat of the Infected which are controlled by a computer controlled 'Director' (AI D). The AI D controls many variables in the map, from weapon placements and types to when a Tank will come ruin your day. The AI D doesnt simply randomly throw hordes of enemies at you but actually evaluates your progress and situation and will tone down the action to give you a breather, or crank up the numbers to provide you with a intense encounter. Players can play Campaign in single player mode where bots will fill in the rest of the team, and do a pretty good job too , or play with friend online in co op mode.
Versus mode is where the player gets a chance to play as one of the Infected. In Versus mode one team of 4 players will play as Survivors (bots may fill up empty slots), and another 4 will play as the special boss Infected (bot dont fill up empty slots, respawn times are reduced to make up the lack of players). The Surivivors must try get to the safehouse of that map , and the Infected must do everything they can to hinder and eventually stop the Survivors from escaping, when the Survivors either make it safely or are stopped the teams will swap roles and points are tallied based on Survivor performance, when both teams have had thier turn as Survivors the next map of the Scenerio is loaded.
The biggest problems for the PC version of L4D would be the poor multiplayer server browsing tools, not that L4D's matchmaking is a complete mess... its just PC players expect more, especially from Valve who has spoilt the PC gamer with the excellent server browser used for CS:S, DoD:S and TF 2 amongst others. The matchmaking system is simply too random to be considered good, one match could be great with a low ping server and the very next time you could get a nightmare of a game.
All in all L4D as it stands currently , is potentially a great game with the right circumstances... it has all the ingrediants that make a game fun and enjoyable, but relies too heavily on getting the right players together on the right server.