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:

If you choose the option, a new Pen opens up with the custom build inserted. You'll also see some HTML and CSS, which show you which features you chose in red (not supported) or green (supported). Of course you could remove that HTML and CSS and just work with the JavaScript build directly if you wanted. Or use that Pen as a resource for another Pen!