Unfortunately, these interface changes won't necessarily be exposed on non Nexus devices -- meaning pretty much the entire universe of Android hardware. You'll still be held hostage to whatever interface the manufacturer wants you to look at if you're not using the developer edition of a phone.
It's unclear how these design changes could affect interfaces like HTC's Sense or Samsung's TouchWiz UI, but we're not holding our breath for substantial KitKat additions to those UIs. Third-party manufacturers will likely continue to push interfaces that match their brand, and for companies like Samsung that means gaudy icons and blue everything.
Never carry cash again
As leaks suggested, Android 4.4. KitKat will enable Google Wallet right off the bat -- without the need for your carrier's approval. And because Android is actually storing your payment information within KitKat, you won't have to rely on NFC security elements to make a transaction.
But that's not all: Not only will you be able to manage your credit card information from within the Settings panel and more easily pay for items anywhere NFC and Wallet is supported, but you'll also be able to send money to friends without annoying transaction fees.
Google enabled the payment settings from within the OS to help people on Verizon, T-Mobile, and AT&T take advantage of its Wallet offerings, as those three carriers have banded together to use Isis as the default NFC-payment platform. We won't know more about how Wallet works in KitKat until we physically have an Android 4.4 device in our hands, but at first glance the update looks like a win.
Okay, Google Now
How you access Google Now varies from phone to phone. Do you hold down the home button? Do you swipe up from the bottom of the screen? Do you shake your phone vigorously while shouting at it? Hoping to simplify things a bit, Google has put Google Now front and center by adding it to the Home screen. Now, instead of entering in some variable command, you can just swipe from left to right to quickly access Google's handy service.
The new OS also supports launching Google Now by saying, "Ok Google" while on the homescreen. It should be a helpful feature if your hands are tied up, or you would rather dictate than type. It's kind of like the Touchless Controls on the Moto X, but with a few more hoops to jump through. Unfortunately the functionality is currently limited to the Nexus 5, so your current phone probably won't get the feature. (But your next phone might!)
Also, Google Now can now access apps when it exposes information in its search results. Let's say you're looking for a particular restaurant. Not only will you get the usual Google results, you could also get an invitation to book a reservation via, say, the OpenTable app. This function has to be supported on the app level, but suggests an exciting new expansion of Google's digital assistant.
Indeed, by enabling easier access to Google Now, third-party manufacturers like Samsung won't be able to hide it behind its own proprietary services. (Ever try to get to Google Now on a Galaxy S 4, only to be defaulted to S-Voice?) Google just wants you to stick with it for all of your queries.