The new Android N developer preview has some important new features, but also a lot of little changes. For example, a new package installer UI. It's not dramatically different, but you can see the new one above and the old one below for comparison.

The first screen with permissions is not much different, except for the button placement at the bottom. The installing screen moves the progress animation to the center and adds a little Android logo. The confirmation screen has a checkmark and "App installed" text in the center as well.

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Package installer in Android 6.0.1

That's the extent of the changes as far as we can tell. You'll only come across this UI when sideloading an app, but for some of you that'll be quite often.