I wouldn't want to substitute the best LW posts and comments for equally time consuming wiki contributions. But many of us are incapable of writing the best posts and comments, but I would guess, could do a decent job of making wiki contributions.

And like SoullessAutomaton said:

a well-structured wiki is more discoverable and accessible

One other hueristic I was using - commenting on LW is more fun because it is social, and yeah, the karma helps too. In this context I think this is (weak) evidence that the wiki will be more subject to a collective action problem. Then again, Wikipedia exists.