First, the new lock screen is the best lock screen if have used on an Android device so far. It is simple and the ability to display notifications adds a great deal of utility. It is incredibly useful to see what notifications you have without having to swipe down from the top of the screen. Straight from the lock screen you can dismiss them by swiping them away to the side or jump into one with just a quick double tap.
Accessing the tablet normally has never been easier. A simple swipe up from the bottom of the screen leads to the home screen. The camera is also available via a quick swipe from the right. This is much simpler than the previous stock Android locks screen and more along the lines of what the OEMs have been doing with their skins. Hopefully this will lead to a more unified experience and lead OEMs to use the stock lock screen.
Next we move to the new notification shade. The notification shade has long been one of Android's greatest features. In my opinion it has only gotten better. Google has rethought where notifications should be displayed. The new format allows for notifications to fit into the rest of the OS. Previously they felt like they were a different part of the experience. They now hover over whatever is on screen. This allows them to fit in with the experience and feel more like part of the OS.
Another great enhancement to the notification shade is the way you access quick settings panel. Previously there where two ways to access quick settings. You could either tap the icon in the top left of the notification shade or swipe down with two fingers. This has been simplified to just one extra swipe down from the top. This makes for a smoother experience.
The settings menu as also been overhauled and now feels more like an app than part of the system. It also makes better use of the available screen real estate. It will put options side by side in landscape mode on a phone and always displays them side by side on a tablet.
Finally, with Android L it's not the big UI changes and back-end overhaul that make it great. It's the little things like displaying the time remaining until the battery is fully charged on the lock screen and subtle animations when touching certain UI elements. Google put a lot of thought into animations. I really like that apps open by sliding in over the previous screen that is subtly tilted back. There are tons of these subtle UI touches that greatly improve the experience.
Android L seems to be the start of a new era for Android. Ever since it was created Android has been considered a second rate OS when it comes to design, but Google has changed that with Android L, bringing Android up to design standards. I can't wait to see what the full release in the fall will be like whatever they decide to call it. Be sure to follow as I will be posting on new features as I find them.