On the one hand, sustainability is a relatively new topic, and collectively we are just beginning to learn about it; there's not much we agree on yet.
On the other hand, "sustainability" is a popular PR buzzword, so marketers have confused the discussion with whatever will sell their products.
When someone asks a question, their assumptions are likely to be wildly different from the assumptions of other users on the site. For example, when someone asks "What's an effective design for sending sink water to a toilet tank?", I want to answer "don't - use Humanure". But I recognize that many people aren't ready to make that change in their lives.
Other sites that deal with sustainability are typically in a discussion forum format, instead of Stack Exchange's question-and-answer format. I really like SE, but it's not well suited to a community with such varying premises.