Khadgar and Wrathion are a major part of the expansion. So is Tyrande.
There will be a big bad, but like Mists of Pandaria, it is something we will discover later on.The initial focus will be on the various local problems.
The Dracthyr will be able to choose a faction but unlike Mists it is not a philosophical/faction rivalry fueled choice and purely practical in nature.
Some small Changes will happen in Kalimdor and EK. It will focus on certain NPCs and the future of the Forsaken but will still focus on the new continent of the expansion.
We will learn more about Galakrond, Proto=drakes, Giants and certain choices dragons must make.
There will be a pre patch event for two weeks. Uldaman will become a level 60 dungeon for the event. We will be able to play the Evoker class then.