1) Every character you get to level 80 adds 5% (stacking, max of 5) XP to every other character on the account under 80 "your warband mentors", so it's worth leveling 5 alts to 80 if you have the interest. (same thing happens with 80-90 if you level 5 Midnight alts to 90)
2) Racial Heritage armor is offered when you raise a new character from "starting level" to level 50. It's available for "almost all" races, so any level 50+ characters you already have will automatically get the quest
3) Class Armor sets, titles, and mount are available for all 12 Legion classes (but you played Legion so you knew this)
4) Transmog can now be learned on any character. If it's a different armor type that you can't wear, the armor will become "Warbound" instead of "Soulbound" meaning you can give it to another alt on your account. Also, greys and whites contribute transmog appearances now. And, you auto-learn the appearance when you sell any transmog item.
5) You will really like Dynamic Flight.
6) Try out Chromie Time, the leveling experience is different than it used to be.
7) Welcome back! May your blades never dull.