The main things is the extra features you get. The much bigger hard drive means you can store up to 250/500 hours depending on what size box you get, as opposed to your 80 hours at the moment. The box overall is a lot faster too, mainly when using On Demand, menus etc.
The best thing to do is watch the YouTube video i've linked below.
http://youtu.be/KBuFkSUlg7A