JSON - The new web API resources provide JSON, as opposed to XML.
Changes in URLs - The urls for character and guild profiles is different, allowing you to specifically request which datasets you want. API resources will be specific to the regional battle.net site they exist in and will be prefixed with "/api/wow/"
Documentation - With the API comes documentation on what data is available and how to use it. Be sure to read up on what is available and what other developers are doing with it by frequenting the Community Platform API forums.
SSL - If you make authenticated requests to the API, you can do so using SSL.
Auction House Data Dumps - Auction house data dumps are available for each realm that contain the then-current snapshot of auctions.