1. The stagger system I dislike. For most battles, it is fine but for a lot of them, it causes pacing issues. Like a lot of the enemies take almost no damage (or have some resistance to damage) until you stagger them. So you are taking like 2-3 minutes of boring (and slowly) damaging an enemy until you stagger them and the fight ends right there.
Also (for boss battles especially), once you stagger the boss, the battle almost ends right there.
So there is this weird pacing issue in fights. I like fights where you don't have to stagger to defeat the monsters.
I think a lot of the balancing in FFXIII got screwed up due to the stagger system (I do love XIII-2 but this also applies to XIII-2 sadly). They should have just ignored it (or at least made it so that damage didn't increase so dramatically from unstaggered to staggered) and it probably would have the game a lot better.
One of the boss battles that was done right in FFXIII is the final boss in Titan's trial (I forgot the name) and part of the reason is the enemy didn't have stagger bar (well it did have it but it was at 999 I think, so you won't hit stagger on it but you will gradually increase the damage you do). The pacing of the battle was better in that fight than the usual "do almost no damage until you hit the stagger limit and stagger the enemy, then for 20 seconds, do 10x more damage until stagger ends, then do no damage until you have to stagger again, rinse and repeat).
2. I forgot that accessory combos were even a thing in the FFXIII (it adds a bit of customization to the game which is nice). I had a lot of fun grinding early in the game and maxing out a few accessories.
3. Speaking of that, a lot of the abilities and traits on items aren't worth it. For a lot of the weapons, you usually just go for stats (like a 700 attack and 700 magic weapon is much better than a 300 attack/magic weapon with ATB Charge II). There are a few exceptions where abilities on weapons are useful (the debuffing wand for example) but for the most part it's better to go for one with the highest stats.
Small things like balancing the stats and abilities better would have made FFXIII a lot better. Also I dislike that Libra costs TP to use (instead of using TP for cool stuff like summons, I find I just want to Libra everything 2-3x just to make sure I have 100% completion for my Enemy Intel in the Data Log). (Also having to use Libra 3x just to find out all of the enemy's details is also silly too.)