[antinescience]

Not only trust, but honesty. It's a lot harder (I would wager almost impossible!) to operate in a flat organization without everyone being willing to both be honest and receive honest criticism.

The company I work for is about 30 folks strong and there are, for all intents and purposes, only two levels of hierarchy. It could probably sustain being completely flat, but I'd fear that it'd fall apart relatively swiftly due to people being unwilling to both bring others to task and be brought to task themselves.

Plus, there's something to be said for decision by committee severely obfuscating progress if consensus can't be easily reached.