Joylette Portlock, Don't Just Sit There - Do Something!
Most significant development: Honestly, the most significant development is the willingness of more and more public officials who accept climate science to finally say as much, and call, loudly, for action. President Obama's climate speeches and plan are the best example. It's a testament to the amazing amount of outreach that has been done on the subject in the past many years, and it bodes well for the future. The more leaders we have calling for action, the easier it will be to remind the public that climate change deserves their attention, too.
Most important lesson: This answer is not terribly academic, but I recently saw organizing guru Marshall Ganz speak about social change, and a lot of what he said really seemed relevant. If our goal is to change people's actions, then we have to create a way for people to identify with those actions. I'm not saying we need to change people's beliefs--the opposite, actually. We must, instead, affirm the things they already believe, as they apply to climate.
Right now, we suffer from an image problem, where people insist you must drive an electric car, eat vegan, hate electricity (except for the aforementioned car), wear nothing but hemp, and wax poetic about the plight of toads in the Amazon at dinner parties (vegan ones, presumably the only ones you get invited to) to properly consider yourself an environmentalist. We need to expand the definition, make the tent bigger, and remind everyone that conservation is a value we all (well, most of us, anyway) share. Once the identity--that, say, of the everyday environmentalist--has been uncovered, involvement and action on the part of the public will follow.
In a nutshell, instead of saying, "Hey, American People, you're doing it wrong!" it's probably a lot more effective to say, "You know as well as I do that we've got to fix this. Are you with me?" As it applies to my own work at Don't Just Sit There - Do Something!, where we use humor and the short video format to make climate science and news more easily digestible, there are so many exciting ways we might try to invite people to see themselves as supporters in the climate movement! I foresee at least one new public engagement campaign in our near future.
Biggest challenge: I'm surprisingly upbeat, like I said--momentum may once again be on our side. But I thought that in 2007, too. So, I would posit that our biggest challenge is in consistency and staying power. We still have to work as hard as we ever have, and work together and more strategically than we ever have, to see the job done. Even though we've seen the first inklings that we may be breaking through the denial, misinformation, and complacency, there absolutely will be pushback from those protecting the status quo. If we let up, then that pushback will certainly undo the gains we've made. And we don't have time for that.