Not just the macro environment of Mother Earth, but the micro environment of your meeting space. I know, I talk about this a lot, but I just saw the photo of the Catalyst Ranch hosting EventCamp 2011's annual meeting on this The Conference Handouts blog, and I wish more than ever I'd been able to be there. (And then I clicked through to see more pictures, and yet more, and really, really want to go to this place, even if I couldn't make the conference.)
Poster Patty Keller shares some good thoughts gleaned from a presentation by John Nawn about conference design, including this question I wish every planner would not just ask, but do something about: "Is the environment stale with buzzing florescent lighting and rows of chairs or does it provide a sense of creativity and learning?"