Requirements is always the problem: capturing and realizing user-desired system behavior in a form programmers can build from. Unfortunately big design upfront tries to nail these down at the beginning, an Sisyphean task because almost no user knows exactly what they need. Agile development should instead not try to capture every requirement, but get a few big things right to be usable and organically grow incrementally with continuous feedback from users throughout the development process. (It's an anti-pattern to stay incommunicado from users for too long or only involve them at the end for UAT.)

Formal requirements in the form of rigid specifications and documentation in waterfall development is useful in high risk, well-specifiable projects such as a nuclear reactor SCADA control system.