Sometimes, I get the feeling that we're all in the story of the "blind ones and the matter of the elephant". Design UI first? Design data model first? Design code first? Write documentation first? Write tests first?

How many things can we actually do "first"? All of them are important. Writing documentation tells you about what you might need implemented. So does designing UI, but that might still leave out what users need. Which means we probably don't know enough about the data model anyway. Or the code.

So ... here is one more blind person's view - iterate as quickly as you can on everything.