Most disappointing aspect is easy: it is completely redundant.
Really, what was the point of it all? The ST was compressed into one movie already: Return of the Jedi. So I guess I have to give Johnson credit for realizing that when he made TLJ and killed off Snoke - essentially saying “we already killed the Big Bad Guy 2 episodes ago remember?” TROS undid this and remade ROTJ - thus undermining Luke’s character arc, as well as Anakin’s (for those who consider the PT canon). Rey is now just Luke…only better (or worse - since Luke defeats evil with Faith/Love, while Rey uses lightning). Thanks to TROS the entire ST is just a reboot of the OT. NO POINT!!
I had more to say, but I’m just exhausted by Star Wars right now.
EDIT: the second most disappointing aspect is all the OT.com members who have said they aren’t sure if they like TROS or not and are going to see it again! Many of them are probably paying for Disney+. How about you don’t give the huge corporation any more of your money and just wait until it comes on Disney+ to decide if you liked it or not?