I look forward to killing the zombie horde with three of my friends but I can see some problems with it.
1) The levels look very repetitive. I know Valve say they have randomly placed zombies, but it looks like the game involves running forward, killing stuff, getting ammo, repeating. There doesn't seem to be anything to break up the gameplay such as Half Life's puzzles or Halo's vehicle sections etc that stops the gameplay from becoming stale.
2)Lack of a storyline. I need motivation or character development in the games I play, otherwise I don't like to continue playing. Survival isn't enough motivation for me to keep on playing, and four characters randomly meeting up and then going and killing zombies isn't very good character development. I could be wrong here, I suppose I need to read reviews to learn more about the story.
3)This games singleplayer is it's multiplayer. Unlike games like CoD4, Halo 3 or Gears of War I can't spend around ten days on singleplayer and then go and waste days on multiplayer. I can only spend 30 hours (tops) on multiplayer before I've beaten it and don't have any more reasons for playing it. There seems to be no replayabilty unless certain parts are really, really fun to play out. Looking at the gameplay, I doubt this.
Unlike other games *cough*Fallout 3, GoW2, Mirrors Edge*endcough* I'm going to need to do more research before I'm sold.