I like approaches described by @AlexanderTD & @lechlukasz particularly. I think focusing on more substantive, quantitative issues will be most fitting for a site titled "Sustainable Living." I like good agricultural tips and housekeeping tips as much or more than the next person, but let's fall back on the definition of sustainability here. How much will my net impact to the ecosystem be if I use cleaning product X or Y? Negligible. What about if I drive my car 50 miles a day vs 5? Significant. Vegetarian vs. meat? Probably a big impact, but I'd be quite interested in seeing some numbers. I think if the site doesn't focus on the comparitive significance of factors, it'll get lost in the noise of trivial issues. As such I respectfully disagree that the discussion of plastics is necessarily pertinent - my consumption of plastics accounts for a pretty small percentage of my overall environmental impact.
On the positive side - I would think it easier to build a base of concerned citizens on here than on most SE sites. Sustainable (I almost cringe at the word~ I kind of prefer "efficiency" since it lends itself to quantitative analysis) personnel are pretty easy to locate job-wise in academia, government, non-profits, the corporate world. I think there's a real opportunity to provide a mostly-scientific exchange of ideas amongst personnel in influential positions.
I think the site should seek out "meaty" questions focusing on core issues: energy production, transportation, building efficiency, food (to some extent), and start to politely redirect gardening & housekeeping questions and tips elsewhere.