As for yarn choices, I'd like to see a wide variety of options for all sorts of knitters and crocheters. Different people like different things. As this moves forward I'd like to see more kits, yarn batches of every weight and fiber, etc. I'm in the Writing Community, and we do everything from ballpoints to fountain pens, starter pens up to the most expensive high-end pens, ink from your basics to your specialty imports. Exclusive color options to the sort of thing you can pick up at Staples.
What I look for in this community is the same sort of range of all the possibilities, and I think it will be most robust if it caters to the full range of needs. The more limited it is, the smaller the community will be.