On January 30, 2018 Battle for Azeroth was made available for preorder, coming with the unlocking of the four first allied races. With the announcement of the Collector's Edition and release date for Battle for Azeroth on April 5, 2018, Legion was the first expansion to be offered for free with a preorder. Battle for Azeroth is the first expansion to launch in the Americas, Europe, Taiwan, Korea, and Australia/New Zealand at the same time. Battle for Azeroth became the fastest-selling World of Warcraft expansion ever.
That's mostly it, but it's linked to the fact you can transmute ghost iron bars into trillium bars, which can be transmuted into Living Steel. So ghost iron is really really useful for sky golems and panther mounts, because they require 4 Living Steel's, which means 24 trilliums or 240 Ghost Iron bars, if you wanna go that way instead of farming the trillium itself!
Mythic+ rewards will also go up by 30 item levels; however, during the first week of Season 2, rewards from end-of-run chests will be capped at Mythic 6 quality (item level 385), and the chest you open next week will contain Season 1 rewards if you do a Mythic+ run this week. (So, for example, if you do a Mythic 10 during the final week of Season 1, you’ll get a Season 1 item level 380 piece and 60 Residuum from your first Season 2 chest.)