Yahtzee I've been reading your reviews since you started last year and usually I just read the forums but your feelings about this game made me feel the need to register and comment.
I agree with a lot of what you said: the annoying amount of friendly fire in the game because the Survivors are dressed similarly and look a lot a like the generic zombie hordes at first glance. And I'm sure it won't be long before the same 4 maps get old. And playing as a zombie sucks since you die almost instantly and can very rarely actually get ahold of one of the Survivors long enough to actually do real damage to one of them before BOOM headshot you're dead.
But while I agree the campaign's difficulties remain uniform throughout the chapters, I'm disappointed you failed to mention the end of campaign, zombie zerg-rush finales which are intense and incredibly challenging on Advanced and Expert.
Sure some are easier than others: the Farmhouse finale is a joke everyone just needs to go to the barn loft and can defend every point of entry in there easily. But other level finales, like the Boat House, are sadistically intense as you're assaulted from every direction. And while there is no Megazord boss zombie at the end of campaigns like you wanted, when the director gets sadisitc enough to throw TWO tanks at you, I think that should technically qualify as one
I also commend Valve for making this the first true FAST-PACED FPS game that requires and encourages good teamwork. Prior to this there really was only Counter-Strike and Battlefield, where half your team played it like a Quake or UT fragfest, or Rainbow Six, which some missions slowed down to turn-based strategy game speed. Kudos to them for finding a good middleground compromise like this that is both mindless fragfest and squad-based teamwork shooter.
I hope Valve continues to support this and offer more downloadable campaigns and new weapons and zombie types down the road. I'd hate for the game to die (lol pun) prematurely because of it's lack of variety when it's design and pacing is so excellent.