The more I think about it, the more obvious it becomes that wikis have been successful precisely because they appear to be simple to write. Eventually when the wiki becomes large enough, the guidelines become sufficiently tedious that anything you write won't be acceptable on the first pass. The appearance of simplicity remains though, because the mechanics of the process are straightforward. That's what I see in wikipedia.
Now, if we apply this principle to semantic wikis, it is essential that they retain the appearance of simplicity, even if they become complex beasts. It is far easier for complexity to appear when authoring semantics, because of the number and nature of links that appear between concepts. It will take a correspondingly longer time for hand-authored semantic content to become coherent, consistent and complete. However, making it simple to author semantic content, even incoherent semantic content, is essential if the wiki is going to retain its contributors.