I would say the main defining feature of Post-Modernism is its skeptical regard of, and general lack of confidence in, all the social institutions - read: memes - of Western Culture.
I would also say that it has been extremely useful, in fact I think the disillusionment at the base of Post-Modernity is what allowed the counter cultures of the 60's to flourish, it burgeoned the Civil Rights movement, and even though it produced some ugly design, in the realm or architecture it allowed for people like Buckminster Fuller to come tot eh fore, it also paved the way for Green, Environmentally friendly, design concepts.
I think the Environmentalist movement as whole is perhaps Post-Modernism's clearest offspring.