There are 2 things I will miss in shadowbringers as AST:

1.) Card Juggling
Having to juggle multiple cards (drawn/held/royalroad effect/crown) was fun. Its a fun part of the cards under the current system. Probably one of the only things that make it fun currently. This game is directly replaced by managing seals which has a lot more player choice in it though. So Theres not a lot to complain about here, but I will miss it.

2.) Extending buff durations. Probably one of coolest parts of Ast especially in 4player gameplay. Doing big dungeon pulls and pulling an Asp.Bene/Asp.Helio/CU/E4E -> Time Diulation -> Light Speed/Cleric Stance -> CO -> Gravity Spam is one of the most satisfying combos to pull off in 4mans with really really big pulls.
This hasn't really been replaced by anything so much as its just been flat out removed. A sad thing for the sake of balancing healers but it was quite fun.

All the other AST additions and changes are very good though.

*friendly reminder that balance changes (potencies/mpcosts etc) shouldn't even be a thought in our minds at this point. Job actions are about the gameplay.*