With Windows Phone 7 featuring such a beautiful UI, there has to be a level of quality present in application appearance and user friendliness to match the core system, which is entirely different to the competition. However, there are a handful (or more) apps on the Marketplace that aren't up-to-par.
Jeff Wilcox, better known for the 4th&Mayor FourSquare app for WP7, has published a Metro UI guide for developers, which is beautiful in itself. Covering the factors; ease of use, simplicity, discoverability, conformance and more, the guide is a must read for any WP7 developer or even iOS and Android developers to gain an insight into the Metro universe.
Almost simply listing what to do and what not to do, Jeff explains the steps that are required and calculations that need to be made for your app to succeed in the Marketplace with both sales/downloads and highly rated reviews. Also, it's always nice to have a resource available for those who wish to port the UI to other platforms, but mimicking the appearance of WP7 is one thing, replicating the user experience is another.