Deciding what you want and writing that down without seeing it first is incredibly difficult. Learn how to use simple stories as a starting place.
Everyone assumes one way but miss out on a key consideration. There's more than just tasks here; there's people.
Everyone on your project has a role, but this role is commonly skipped or sometimes incorrectly turned into a committee.
Need to decide between Waterfall and Agile? There's a consideration that you don't expect.