There are three raid tiers coming in Legion! Two raids are are coming with release, another raid in Patch 7.2 and one more raid in a later patch.
Legion has a lot more patches and different things planned,
along with different sized patches!
Patch 7.1 will go on the PTR very quickly around the time that Legion is shipping. It will be released pretty quickly after Legion comes out, containing a mega dungeon.
Your loot specialization will control what legendary items can drop for you in Legion.
Improved social features won't be in at release, but hopefully soon afterwards.
The artifact skins that aren't in the UI yet don't have final sources yet. They will probably be rewarded from achievements, likely tied to new content as it is released.
Another stat squish is coming in the expansion after Legion.
There will be a new animated series in Legion called Harbingers, similar to the Lords of War series in Warlords.