Nightshade: Petrell has the right of it for the most part; I think I like it a bit better than he does (I'd call it at least an 8) and think that the zombies can be a bigger problem in the early days than he's suggesting, but other than that the virtues and flaws are pretty much as he indicates.
As I understand it, there is a new zombie type (crawlers) in game that did not exist when I played so things might've changed a bit but biggest problem remains. That is the fact that zombies and npc's just stands in place until they hear noise, see you or are engaged by you. If zombies would wander around even a bit and looters could actually loot a places they're supposibly looting and possibly then leave, game could be lot more interesting. I think it's more of a bug/oversight but zombies further away that heard noise just stop eventually instead of continuing to wander towards the location of the noise. Probably due to instead of giving them a waypoint to location of the noise they continue towards the noise only while it lasts and stop as soon as it stops. Would've been in serious trouble few times had they continued on coming.
Even the scripted events on maps are quite boring (the ones where a faction assaults a map while you're there) as as soon as combat is over, the survivors will again just stand in place. The standoff maps are also akward as nothing happens until you make noise and all hell breaks loose. The NPC's should be scripted to break the standoff eventually even without player's interference (and, if player doesn't stop them, eventually leave with all the fancy loot).
In general, the biggest problem is that world is just way too static to be believable.
Edit: Now that I think about it, one of the reasons zombies were so easy for me was that my main char was fast melee build with 12 ap. This ment I could swing most melee weapons 3 to 4 times and later even more with a perk (6 attacks with electric batton will kill or knock out ANYTHING, even fully armored soldier). Melee creates very little noise so most of the time only the zombie I was attacking reacted to me, even then only if it survived my intial assault. Any other kind of build probably will have harder time in combat early on. Ranged build may excel in late game but early on you're short on ammo and, with the exeption of bows and crosbows, they tend to be noisy and attrach unwanted attemption. Pistols you have early on aren't that effective either nor do they have much range... well, firearm range is rather pathetic in this game but later weapons do pack quite a punch and are, generally speaking, superior against armoured opponents.