The way we treat animals. I think in the future a lot of the species we take for granted will die out, and eating meat will be a luxury many won't be able to afford, so people will look back at how wasteful we were with our food and resources, and if only we hadn't farmed so intensively, maybe our planet wouldn't have ended up like that... In short, they will hate us!
Maybe by how much kid's are getting pregnant, or by how the music changed rapidly, from country, to fist pumping music. I think that's a big change, and I think the people from the future will agree.