As hard as it is for some to understand (and I don't necessarily mean you) altoholics do like to get shiney new stuff. Especially after watching raiders/PvPers getting stuff shoved down their poopers year after year.
As to what it will bring, BW said they planned to include new emotes, abilities, gear and (maybe) titles (not sure on this one).
EDIT: and of course different species, although as of now it's everyones guess if they are species that are atm unplayable (like Ithorians) or if it's just going to be new species/class combos of the existing species. BW has been a bit cryptic about that.