I use my nexus and the constant grey puts me off. Very little contrast, very little distinction between what is UI and what is scaffolding. If you're going to do a flat design, the elements have to separate themselves from one another -- things have to be distinct. This is a great example of a flat email interface with basically the same layout as Google's client: http://www.vanschneider.com/work/mail/ everything being #cecece and shifted one direction or the other isn't.