Big changes are happening from visual style, animations and such to hardware perfomance because it will actually be 64-bit, just like Apple's latest iOS. That means a lot, cause most computers aren't even 32bit yet and yet something we hold in our pocket is.
Google back in June, with the first time giving developers and opportunity to test it out on their own devices, such as Nexus 5 and Nexus 7. The design, yes that where it comes to since we actually do look at our phones all the time, design language is called and referred to as Material design (while the Windows Phone design is called Metro, or sometimes even modern). It all looks flat and there are some handy improvement such as accessing your notifications even from the lock screen and actually interacting with them. Imagine a situation in which you get a message on Whatsapp and you want to take a quick picture, well you can do it from the lock screen.
GPS apps work on Android L, well the same as they did on the older versions, only the will probably look better since they will get updated to this new interface Google is pushing to our eyes, and yes we like that since it will be a lot easier to look at. Modern icons, bigger typography and clean colors, that's a good design in short.
All of Google product apps will be updated and modernized, can't wait to see what gMail looks like! If you are still wondering about the name, if it doesn't come out sponsored as last one it will probably be Lollipop.