Android 6.0 System UI Tuner
In the earlier developer previews we had access to something called the "System UI Tuner" which allowed users to add a battery percentage to the battery icon in the notification tray, remove icons you don't need (like location, or other icons) and make changes to the entire system UI in terms of the notification bar and Quick Settings.
We thought it was gone, but it's still available. Simply long-press the gear-shaped settings button in the pulldown bar, and hold for a while. It will spin, and unlock the System UI Tuner controls in settings. You'll now see a wrench near the settings button, letting you know you've unlocked additional controls. From here customize the pulldown bar, quick settings can be rearranged, and more.
This is just a few slides showing users some changes, settings, options, and new features of Android 6.0 Marshmallow, but surely more are hiding underneath the skin. There's a split keyboard mode for the keyboard on tablets, and there were tons of advanced features in earlier Android M previews that are now missing, but could return in future updates. Many that were in the M preview are now in the N preview and work better. Later in 2016 we could have a dark mode, night mode with the screen going red to prevent eye strain, split-window true PC-like multitasking and much more. For now, only a few options are available in Android Marshmallow.
Stay tuned, as we'll be adding to this list as we find more changes, useful features, or new additions to Android 6.0 Marshmallow.