Journalists often mention that Windows Phone 8 runs the NT kernel. What is the significance of the switch to the NT kernel for end-users (i.e. better performance, more hardware supported, etc.) and application developers?
The significance for end users isn't that high. It's not something that is visible for end users. It does however tell the story of WP8 that its foundation is completely different from WP7. The shell showing the user interface however stayed the same for the end users, so they still see a comparable user interface (apart from the changed that are made to WP8 with the new Start screen etc).
For developers it's a completely different story. Whereas on WP7 you were building Silverlight apps, you're now building apps in C# and XAML (almost the same code, different name) but the APIs to communicate with the operating system are totally different. These APIs are also party shared with Windows 8 so you can share large parts of code between Windows 8 store apps and Windows Phone apps.