Play play play; playable playable. Run some drafts, figure out what archetypes need help/need less help/ need to be more interesting or more unique. Balancing the commons needs to be not just across colors, but also archetypes; it wont help if every color has the same number of good commons, and three colors' worth of them say "draft enchantment deck!" The commons thing is more something to keep in mind while tweaking cards for the many other reasons they will need to be tweaked, than a "task" that is done at some point.
As far as deciding "communally" how development will change cards; I think a different approach should be used than the piece-mail voting sytem. Maybe having interested individuals submit "portfolios" of multiple changes and describing the goals of the changes. Then people can discuss them and suggest tweaks (such as the community indicating that it is especially important to them to keep a certain common in a certain form), and hopefully a consensus can be reached for a certain portfolio. If there are substantially different portfolios they can be voted on, (or specific lines-of-reasoning can be voted on, similiar to the way the spell mechanic was decided.)
I'm not sure how much the NGA community is actually interested in creating a playable set. A lot of mechanical issues I've mentiond or seen others mention in this thread and other threads don't appear to have an impact on the criteria people set, the cards they design, or the cards they vote for. (How many non-rare flyers do we have now? I haven't been paying attention.) This is shaping up more like "From the Vault: Maraka" than something someone can draft; that's fine if that's what the community wants. If it isn't what the community wants, though, development is probably going to have to make some radical, feather-ruffling changes from what has been laid out even at this point.
I guess I am just offering this criticism as food-for-thought; there are still a lot of cards to make, and maybe the right cards to fix the mechanical issues just haven't been thought up yet.
EDIT: Also prepare to not let your feathers get ruffled.