Since its inception, Windows Phone has had a bold visual design language defining the core experience of the OS. Initially called Metro, Modern design gets its influence from the work of Massimo Vignelli, who famously made the NYC Subway maps, and the school of Swiss Style started in the 1950's. At Microsoft, the lineage starts with Windows Media Center, up through the ill-fated Zune HD, finally spearheading the core design of Windows Phone 7 and Windows 8.
Android though, has never had such luck. The UI has changed significantly since Android 1.0, often evolving around a very loose, almost ad-hoc set of principles. Things began to change once ex-Palm design head Matias Duarte joined the Android team back in 2010. Since then, we have seen attempts at a new language, notably the Roboto font for Android 4.0.