The trailer sends chills down your spine, especially if you've been paying attention to the lore of Warcraft since the RTS games back in the 90's and early 2000's. It shows a certain very important figure in Arthas' life, letting him know the Fall of the Lich King is bound to take place.
For additional information about the lore in this patch, check out our article on the Lore of Patch 3.3. To see more of the dungeons and have a bit more of the story explained to you, check out vodka's trailer.
We understand that the YouTube trailer might be taking some time to load. There's a lot of people trying to access everything at once, and most internet services are getting bogged down. WoW.com is on AOL webservers, so we should be fine, but other sites, include Blizzard's sites, are not able to handle the load as well.