Lots of things: the purpose, the outcomes, the audience, the content, the quality of the build, the functionality, the design.
It’s easy to start with the design, because you could embark on it without knowing the answers to all of the other things. You could choose to ignore everything and just create something pretty and worry about the rest later.
But who benefits from such an approach? Not the client, not the target audience and not the intended outcomes for the website (unless through a sheer stroke of luck). The beneficiary is the designer. He or she will end up with a pretty piece to put in their portfolio. Nice, but not what matters.