In the early days, it seems like there's an infinite amount of things to build. I think there's two important points on prirtizing. If you can, don't do any of them. Don't focus on anything else. When you're tying to make discontinuous improvement, optimize for learning.
David Rusenko from YC alumus Weebly details the story of how Weebly developed one of the most popular website creation and hosting sites on the web today.