
With over 5000 new APIs in the update, the update is certainly huge and will likely be considered Android 5.0.
The Android buttons have become minimalistic, Gmail has a brand new interface, and the icons have all been rounded. The Keyboard has been updated, as has the quick settings notification menu. There are additional do-not-disturb options included, as well. All of Google's most used apps (dialer, Hangouts, etc) will have the new theme. Google says the default Android font will be updated to work across all types of devices, including Auto and TV.
Google has merged the lock screen and notification menu, making the lock screen more interactive, and more Google Now-esque. Google has created transitions and animations that move at 60fps, a nice improvement for the visual elements of the OS. Multitasking has been updated, as well, with stacked cards showing the recently used apps and recently visited web pages.
Performance should see significant improvements with L, as it runs exclusively on Android Runtime with an additional Android Extension Pack for better higher-fidelity graphics when playing games or watching videos.
Finally, Google has introduced remote wipe and improved gestures.
