The Google Now Launcher is mostly a stock experience, but there are a few very nice extra features, which are (obviously) focused on Google Now. First of all, Google Now gets a permanent home to the left of your main homescreen, which is surprisingly convenient, even if you usually use the quick access gesture of dragging up from the home button. The other nice feature is being able to use the "OK Google" hotword from any of your homescreens to automatically trigger voice commands.
As for the smaller features, you can import your homescreen layout from any other launcher, and the status and navigation bars are translucent to really let your wallpaper pop. The app description also gives some features like "easy access to your favorite Google products" and having access to a dedicated search box, but those are pretty silly given that any launcher could offer the same.
Download: Google Now Launcher