An interesting thing about A Mighty Wind is the way it backs away from the documentary framework that was so successful in Christopher Guest's previous films - I'm told it's actually been criticized for this. I guess such a critique comes out of the notion that documentary/mockumentary was central to Best in Show or Waiting for Guffman, but I'm not sure I buy it. Yes, formally these other films adhered pretty closely to the documentary format (although in Best in Show some scenes become pretty implausible as part of a documnetary), but I think what's much more important is the fact that the bulk of the films are improvised.
Improvisation is pretty difficult to pull off in free form. Jazz musicians for instance don't just play whatever comes into their head - there's generally a strong constraining framework, and a very limited musical language to employ within that framework. Another example I like to bring up is the Homeric epics, which were improvised in much the same way - there were specific metrical constraints (not to mention those of the plot), and the poet/improviser relied on a vocabulary of metaphoric constructions ("the wine dark sea" or "rose fingered dawn") to help manage those constraints.
I see Guest's brand of improvised humor in much the same light. Guest and his collaborators spend some time carefully building an elaborate world in which to play - in Wind they design album covers, write songs together, invent a simple plot they can riff off of. Then, they start shooting, coming up with a lot of the most hilarious parts on the fly. But like the jazz musicians, these jokes aren't just free form - structurally a lot of their humor is very similar, so that it almost seems methodical.
I think this kind of improvisation works so well in a documentary is that documentaries are so mannered - they're highly constraining, and improv needs a somewhat constraining framework to succeed. Could this be why some people think Wind is less effective than the earlier films?
I don't think it's that big a deal here, in part because Guest and company have done such a convincing job building their backdrop, but maybe also because the form is implicit after so many trips around the block. It's no mistake that each successive production has less documentary in it - Wind only really has a documentary feel at the very beginning. It'll be interesting to see whether they can pull away from the form altogether in the future.