Let me give you a short answer to a question that deserves a longer answer.
Populism can be useful when the established political parties agree on fundamentals but are failing to ask certain very important questions about the evolution of the economy and society. I think it is fair to say that when it came to trade, for example—and once again, I'm talking about my own country, the United States—the elites of the two major political parties did agree on fundamentals. That's one reason that China's accession to the WTO passed with strong bipartisan support.
To the extent that populism challenges easy agreements and brings to the surface important issues, I think it can be positive. Where it becomes negative is when it takes a pure majoritarian form that seeks to override liberal protections for individuals and minority groups. That undermines a fundamental building block of liberal democracy as I understand it.