Blizzard Community Manager Lore has put together a short video showing off the biggest new features coming in patch 5.4, starting with the Timeless Isle, before moving onto the Siege of Orgrimmar, Flexible Mode, Proving Grounds, the conclusion of the Legendary Quest Chain, Ordos, and more. Do have a look at the official patch preview, as well as checking out WoW Insider's extensive patch 5.4 coverage.
What are you looking forward to the most? WoW Insider's staff are pretty torn between the new raid, and flexible raiding. As Lore states in the video, the new difficulty means that you can bring all your friends along to the new raid, and it's cross-realm from day one, so battle.net friends within your region, be that US or EU, can join too.