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Most importantly: Modernizr 3! It's been a long time coming and has lots of awesome stuff in it. Lots of new and updated feature detects (the whole point of Modernizr).
The design of the new site is wonderful. Clean and fast. The new download area really encourages the construction of minimal custom builds of Modernizr. You can download a custom build directly, or, open the build over here on CodePen!
Here's a quick video: