I've said it before and I'll say it again: version 1.0's Path Companions!

Also an option in system settings to switch the GUI style between the ARR style and 'Legacy style' - that is, the interface art style, ability icons and font from version 1.0.
A 'solo story mode' for dungeons/trials/raids where you don't need to enter with a party and thus the difficulty is adjusted for solo play, but the trade off is you can't get gear drops (apparently something similar is actually being considered for the future).
Mounts having their own unique music tracks (such as the magitek armour having FFVI's Terra's Theme) rather than simply reusing The Rider's Boon for most non-chocobo mounts.