LONDON—One lesson from an unprecedented year of elections around the world is that voters in industrialized countries are particularly unhappy, ready to boot unpopular leaders out of office and making it more difficult for politicians in power to enact bold programs of change.

Rarely have the rich world’s political leaders been so widely disliked. No leader of an industrialized country other than tiny Switzerland has a positive rating, according to a survey of some 25 democracies by pollster Morning Consult. Ruling parties that went to the polls this year largely got a drubbing, including in the U.S. and U.K.