I think the saddest thing that's happened to our world in the past 20 years is...
Sad to me has been the politicization of rational discourse, making conversation about issues important to all of us too much of a chore because people in conversations tend to spout sound bytes instead of considered responses to issues or ideas.
Similarly, I think as sad, if not sadder, is the general shift away from care about others--away from the traits of compassion and decency--and toward identification with small groups, be they religious, political, or cultural.
Saddest of all would be the knowledge that the world I leave for my kids is--so far--not better than the one left for me by my parents.