See Mark's post regarding 7th grade; my intention was aimed at adults, who (for whatever reason) seem to like the 7th grade reading level.

I'm not sure how to effectively crowd source this without getting volunteers for specific (non-overlapping) tasks and sections. I share your concern with the wiki-method, unless each section has a lead. At work we regularly get 20 people to collaborate on ~100 page proposals, but the same incentives aren't available in this case. Copyediting is time consuming and unexciting; does anyone know of similar crowd sourced efforts? I found a few but most still had paid writers.