Official site (also wowlegion.com)
Official Blizzard press release on release date and features
Major new features:
* New Hero Class — Demon Hunter.
* Artifact Weapons — Class-specific weapons of lore
* New Continent — The Broken Isles. Lots of night elf stuff plus the Emerald Nightmare
* Boost to Level 100 — just like WoD's boost to 90; available now if you pre-order.
* Class Hall — Gathering spot for classes (horde and ally together, apparently), with class-specific (group/class?) missions
* New PvP Honor System — I don't care what they do as long as we still get to kill orcs.
... plus the usual assortment of new zones, new instances, etc. etc.