The new user interface is all over with new slick minimal user interface and new pane to view contacts, just like the one you see on Android default contact apps with big rectangular block of contact pic. It does catch the eyes.
Apart from new user interface it doesn’t bring anything more. It however will not mess you with what you are used to and almost every feature work just like before so don’t worry about that part.
Currently the update which is version 2.12.40 is available only on Whatsapp.com and you need to manually download and update, while the Play Store version may be updated in coming few days.