The other is a gameplay trailer of Lament of the Highborne. For those of you not familiar with the quest, you find a necklace when questing in Ghostlands and are told to turn it in to Lady Sylvanas in the Undercity. When you do, after angrily sending you on your way, she sings the Lament of the Highborne, accompanied by some Highborne ghosts. This is one of my favorite quests in WoW and the song is hauntingly beautiful.
The video does more than just show the gameplay footage of Lady Sylvanas singing her lament. It shows the actual story and loss of the necklace while the song is sung. It does not explain why Sylvanas is displayed as an undead Night Elf normally but is a Blood Elf in the video, but I can get over that. Maybe Blood Elf Banshees look like Night Elves for some reason.
The BlizzCon video is good if you want to see the fun of the event, but for me the real treat is the Lament of the Highborne video.