Gharuvy over on the forums posits this theorem: that most instances/raids/encounters are based upon the ability of 25% of the players to "do their job effectively"-- for 5 mans, that's just one person, for 20 mans, it's about five people, and for a full raid it's 10 people. He also adds that it only takes one person to "ruin" an encounter. He then adds, somewhat poetically, that that is the "lament of the raidleader"-- it's too much to expect everyone to be in that "effective" slice of the group, but not too much to expect no one to be in that "ruin" section.
To a certain extent, he's right. People have run Onyxia, perhaps the seminal 40 man raid encounter, with only 10 players (in fact, they've run it with six before). In my personal experience, as long as you have a core group of people who know what they're doing, everyone could pretty much stand there and do the bare minimum, and you'd still finish an encounter without much fuss. And I personally have dragged four people with me through Deadmines on my priest. They were terrible-- one mage pulled aggro constantly, the other didn't sheep, the warrior didn't use a shield, and didn't bother to even switch targets when the mage pulled aggro. And yet I pulled them through it all just because I played my role perfectly and kept everyone alive while doing damage (just in case you think I'm bragging, I've been on the other side of it, too-- I'm in MC when a family member calls, and I'm too distracted to do anything on my Shaman except spam lightning bolt).
And it's true that most raidleaders don't (or at least shouldn't) expect perfection. On any raid, you're going to have a core group of people who know what they're doing, and another set of players who either aren't well geared enough or (more likely) experienced enough in the encounter to play their roles well. Is it true that one person can "ruin" the raid? In some cases it is (ever had a bomb explode an entire group during Baron Geddon?), but in most cases, I think one player's stupid actions will likely get them killed, but not necessarily down the raid.
Then again, I'm a casual raider, so to date, I've never played Naxx. Do you need more than 25% to be on top of their game there? Are the later raiding encounters more difficult in that you really need to coordinate 40 people, or, considering everyone is geared well, can you pull them off when 1/4 of you are great, and the rest of you are just pretty good?