I think the Agile Manifesto is still valid and a great guidance. Also, the principle that between features, delivery date and staffing you can plan for two ahead only, but but not all three.

Unfortunately "Agile" as practiced is almost the opposite in most cases. With traditional waterfall at least people thought about things upfront. With the way a lot of companies do Agile there is no place left for thinking, it's more like mini-step waterfall without upfront design.