"Old enough to know better, Young enough to not give a fuck"
Regarding the auto-combos, this is the first game in comparison to other games that have used this auto-combo system (Under Night In-Birth, JoJo's All-Star Battle, etc), where the second hit can be used as a standalone command normal/unique move for all characters with the use of 6A. For some characters, it is also more easier to cancel either the second or third move of the auto-combo into a desired move of one's choice since the previous game.
The original Short-Hop mechanic can now be cancelled into from an attack with less startup, despite the fact that it ruins any implied overhead setups as it won't cause any jumping normal done from a cancelled short-hop to hit overhead, this new tactic is to mainly counter possible throw reversals. Evasive Actions can now be Guard Cancel Rolled into at the cost of 50 SP. Recovering from certain attacks or getting counter hit normally will cause a counter hit state as usual, only some moves now provide a window of a free Fatal Counter hit, which makes the aforementioned moves more risky than before.
A new system implemented is known as the "S Hold System" (coined by the recent trailer primers as the "Suplex Hold System") which is a special attack which changes its properties the longer the A button is held down, physically shown as a meter on the screen (Skill Boost, SP Skill, Awakening SP Skill then Instant Kill in that order of held-down duration). Aimed at beginners, this may not be as practical for battle, but it still has certain useful applications regardless.
Enhanced Super Moves called Skill Boost Skill Attacks allows characters to perform a much powerful SP Skill Attack by spending 75 SP instead of the usual 50 SP and using two associated buttons instead of one (much like how one does a normal Skill Boost, and is also reminiscent of Enhanced versions of Supers from other fighters).
Another major addition to the game is the addition of Shadow-type characters, which work similar to the EX/Ura-type versions of characters in other fighting games. With the exception of Elizabeth, Shadow Labrys with Asterius, two versions of Sho Minazuki, Tohru Adachi, and Marie, there exists an alternate mode based on the Shadows of the Persona series. Shadow-Type characters are more geared towards the offense as they build more SP as they attack and their SP is carried over between rounds. Shadow-type characters utilize the character's original auto/dial combo from Persona 4 Arena and can use Awakening SP Skills with 100 SP. Shadow-Types are also altered on their stats in comparison to their normal counterparts, as they all have 2000 plus HP and deal 20% less damage overall.
In addition, they gain access to a mode known as Shadow Rampage (Shadou Bousou), which gives them unlimited SP for a limited amount of time and is activated by what would be the Burst command while the character is at 100 SP. In compensation for these additions, they lose major defensive techniques, mainly Bursts and Awakenings despite their larger HP amount. Thus, this makes the Shadow Type a high-risk, high-reward playstyle with an emphasis on skill usage.
Also, it should be noted that in the case of Labrys and her Shadow mode(s), there is indeed both a Shadow Mode Labrys (with Ariadne) and the original Shadow Labrys (with Asterius).
Other changes include lowering the overall damage of all characters in comparison to the last game, and as well as the new HUD somewhat reflecting Persona 3 more (turquoise-like blue) as opposed to Persona 4 (gold and yellow). Also, for the first time in a while, Ultimax like certain fighting games also enables an extra Super Meter bonus on first attack landed after the round's start, where an additional 10 SP bonus is given (even for both characters if they managed to both counter hit themselves and land the first attack at the same time).
Also, exclamation marks not only appear above the characters' heads as usual in recent games for any incorrectly blocked moves, but also during a Negative Warning.