Unlike the iPhone's Siri voice assistant, Google Now tries to anticipate what you need and offer that before you even have to ask. It delivers information based on past search requests, Gmailtraffic and other ways you use Google services. The more you use it, the more it learns. On the Nexus, you can access all that by swiping from the left of the screen, rather than finding and tapping a search box or waiting for a notification.
As for KitKat, the most useful improvement is its new phone dialer. What you'll probably notice first is the lack of a numeric keypad when you open it. Instead, you get boxes showing favorites, frequent contacts and recent or missed calls. It's an acknowledgement that few people know phone numbers anymore. We tend to make calls from contact lists.
