"Flat design also forces you to really care about typography and layout, two areas where web design has traditionally lagged behind its more established print cousin. And on the mobile side, flat design can make it easier to focus on animation and interaction design, as apps like Letterpress and Clear have shown us."

This bit is a good summary. I think flat design on mobile really frees designers up to look into animation and interaction design. Letterpress definitely proves that. It also frees designers up from focusing on detailed effects and stylized UI. Focusing on typography and layout will create much better and more usable applications.