Justin Angel, who recently joined Nokia's developer experience team, has published an article on his personal blog that, over 100 pages, details all "1500" of Windows Phone 7.5's new APIs. The reference article is aimed at developers who want an overview of the changes between NoDo and Mango. 

As Justin explains:

The goal is to inform developers who aren’t familiar with Mango of what new APIs are available. Since there are  “1,500” new APIs we won’t be able to deep-dive into APIs with code samples. We’ll examine the new APIs and their related class diagrams.  Most APIs should be fairly straightforward to professional .net developers who should be familiar with Microsoft’s API style. Each section of this article also contains external links for those wishing to learn more about the APIs, featuresets and their usages.

While your visiting Justin's site you might like to check out the rest of his blog, which has a Metro inspired design, including this post that offers an analysis of apps in the Marketplace, including a look at "cloned apps" and a breakdown of the most popular third party libraries used by Windows Phone apps.