After the Alliance and Horde stopped Hellscream in Orgrimmar, Hellscream was put in prison and made to face justice. However, Hellscream managed to escape imprisonment and with the help of a Bronze dragon, travel back in time to just before the first invasion of Azeroth by the Horde. Garrosh believed if he could stop the Orcs from drinking the blood of the Pit Lord Mannoroth, Hellscream could change the course of history and the Horde would be victorious over the Alliance. The Dark Portal in the Blasted Lands was reopened with a time machine element and the Iron Horde poured through. Both Horde and Alliance travelled through the portal to stop the course of history from being changed.
This expansion saw a lot of change, the newest item was Garrisons where players could build their own and have followers, who they would then send out to do work on their players behalf.
Flying on Draenor was disabled at least until 6.1 patch. Using taxis service is available but you just fly yourself. It came out days before the 10th anniversary of the release of the original. At the start of the expansion, there was a lot of problems with instancing, people could get into their Garrisons and people were complaining. It took a couple of days to settle down but now its near back to normal. Blizzard announced that the expansion has enticed an additional 3.3 million player to come back.
The character models of the original races such as the Human, Night Elf and Orc were updated along with the Draenei of the first expansion. The Blood Elf models are pencilled in for updating and will be later. The newer races such as the Pandaren are up to date and won`t be updated. The graphics look simplistic compared to newer MMOs such as Guild Wars 2. Blizzard countenance the graphics by saying they want low resolution so as many people could play the game as possible.