It turns out that the launcher is powered by the Google Search app and it doesn't work if you disable the app. This is not unexpected: two of the standard features of the Nexus home screen are added by the Google Search app: the Google search bar and the swipe up gesture for Google Now. KitKat brings support for the "OK Google" hotword, so you can trigger Google Voice Search without pressing a button, and adds a new gesture for Google Now: swipe right.
I installed the updated Google Search app and the launcher on an old Galaxy S2 using the instructions from Phandroid.com. I must say that the experience is completely different from Jelly Bean: the app launcher no longer includes widgets and it looks more crowded, the entire launcher can be managed from the Google Search app.
The full-screen Now widget can only be removed if you disable Google Now in the settings or if you install a different launcher.
You can no longer disable and customize cards from the settings. Google opted for a simplified customization feature:
You can now add as many shortcuts and widgets as you want because the number of screens is unlimited. The screens are added and removed dynamically, just like in iOS. You add a new one by dropping a shortcut or a widget and you remove it by deleting all the shortcuts and widgets or dragging them to other screens.
The home screen includes a search box, a Voice Search button, hotword support for Voice Search and a Google Now screen. These are the main features of the Google Search app, so you'll use it without opening it. Removing friction and making the experience more seamless will encourage more people to use Google Now.