The 2.1.0 has go live since 22 May, but there are problems on some realms, making players unable to play right after the patch.
Not to worry, the Blizzard guys are fixing it and all we have to do is wait.
2.1.0 is the most comprehensive single patch released by Blizzard so far. You can expect the patch note to be as long as a thesis (not that long, actually).
Main features of the patch includes the opening of Black Temple, a 25-man raid, which you will face the mighty Illidan as one of the boss. New factions such as Orgi’la, enable Level 70 players to quests and grinding for more rep. Netherdrake quests are available for you to complete, to get the ultimate prize of a epic flying netherdrake mount.
For more information of the 2.1.0 patch, you can proceed to the patch note at World of Warcraft website. It is really too long for me to place it in as an entry.