First and foremost, I'm glad you've got the right mentality here. Too many people see the game as a vehicle for human leet-ness, with zombies as just a mechanism to that action, as opposed to equal and opposite sides. Balancing the game so humans have it tough, but aren't immediately wiped out, is certainly the hardest part of moderating.
Thanks. A quote from our game coordinator, in reply to one of the complaining humans: ">>Assuming humans are the good guys and zombies are the bad guys"
Personally, I don't like the idea of special infected, as I abide by the "KISS" rule (keep it simple, stupid); asking humans to remember which zeds do what based on arbitrary markings in the middle of combat might be a bitch much at times. As such, all human difficulty has to come from the mission design, rather than buffing zombies directly.
Our game coordinator has set a couple of rules for Special Infected: (a) only one new zed type per game, (b) very distinct appearance, and (c) very simple mechanics. We didn't have any problems with the Laundry Zombies in the 6-Hour (LZ's wear two tube socks tied around their head instead of a headband, so fairly distinctive), and so far everyone likes the TAZ.
The best missions are those in which humans are split up, or have to complete multiple objectives to win, with dire consequences if they lose. Missions featuring "sit here and hold" are only good later on, when that's all the remaining human tank can be expected to do. My favorite missions are Day Two missions (first real missions), where we have to find a way to split humans up enough such that 20 zombies can be effective against maybe 70 humans. You can do this a multitude of ways, such as asking them to hold seven nodes simultaneously, having a massive scavenger hunt around campus (bring me back 50 of these Easter eggs within the hour), or by purposefully isolating humans (our mission this semester featured humans crossing a chasm into a safe zone, that were then unable to assist the survivors yet to cross; this resulted in ever dwindling numbers of humans fighting the horde). You can give zombies indirect buffs as needed, such as close respawns and short respawn timers.
The "split up/complete multiple objectives" type is basically what we've been thinking about so far. The "massive distributed scavenger hunt" idea is also a good one. Can you give a bit more detail on how the "crossing a chasm into a safe zone" mission would work?
(Also, side note: we're dealing with a bit more than 90 players here. Try a few hundred for a weeklong.)
As for how to respond to the humans, frankly, tell them to suck it the hell up. This is not a computer simulation meant to display how badass they are; it's a game with two equal and opposite sides. If they think it's getting too hard for them, well, they're more than welcome to sit out. In the meantime, you'll be trying to make the game more fun for everyone, not just a few humans convinced the entire horde exists for their entertainment.
Pretty much what I was expecting. Quite frankly, I think that once they're through with the whole thing they'll actually enjoy the challenge.