The main reason why I found the game to be subpar FF game was because of the battle system and game mechanics. I already mentioned the battle system in my previous post, but just to reiterate, there seemed to be a lot less strategy involved in battles. Paradigm setups are universal, and you can use the same setups on a bulk of the mobs in the game without having to change anything. The AI also makes a lot of poor decisions (ex. rotating strike / spells as a ravager when either your strength or magic is significantly higher, buff / debuff priorities, etc), and you aren't given the ability to micromanage and override the commands. Also, status effects were much less threatening in this game and wore off quickly. I never thought I'd miss Bad Breath, but crippling status effects forced you to formulate strategies, and there was never a situation where I found debuffs to be debilitating at all. As a result, status recovery items and status resist equipment (which doesn't even grant full immunity), were almost entirely useless. Foregoing mp was also a big step backwards, and again took away from having to make up specific strategies. At any instant, you can cure someone who's in critical health up to nearly full, and you can do this for an indefinite amount of time. Also, not being able to move your characters was also an issue, sometimes your characters would all be clustered and if you're about to get hit by a big AoE, there's nothing you can do, whereas in other scenarios the same AoE will be much less threatening because your characters are distanced from each other. My last gripe about the battle system was also that there was no sense of progression at all. You still use tier 1 elemental spells, ruin, etc regularly, even though you've been using them since chapter 1. The only spell that comes to mind that really superseded it's predecessor was Curaja / Curaga compared to Cura / Cure.
As for the game mechanics, there was also a lot of problems. The biggest of these for me, was how restricted you are after you beat the game. Out of the 13 chapters, you only have access to 2 of them (11 & 12) and after you defeat Orphan, you can't even run through the final dungeon again. As for the 2 accessible areas, there is very little use to return to Eden aside from farming turtles and sacrifices, and half the areas in Gran Pulse you can run through in 2 minutes (Suyla Springs, Vallis Media, Oerba). I don't think I need to say how retarded this is. It's a shame too, because areas like Gapra Whitewood and the Sunleth Waterscape looked amazing, but you only get to see them for ~1 hour per playthrough. Things like the monster arena and celestial weapons in FFX, or hunts, NMs, and farming bazaar items in FFXII gave you a good reason to go back to previous areas, and they really made the most out of every zone. Not to dickride FFXII too much, but another thing I really enjoyed about the areas was that a lot of the time there would be an area with higher level monsters which was blocked off that you could return to later after progressing through the game, and there was also a wide array of zones to chose from to level up (Henne mines, Lhusu mines, Pharos Subterrane, Great Crystal), whereas your options are more limited in FFXIII.
The method for obtaining weapons and accessories was also terrible. The sole method for obtaining virtually anything in the game was gil; you didn't have to do high level missions, kill hard enemies, complete an elaborate side quest or anything. To get any item in the game (except charms and elemental rings), the method was the same; farm turtles or sacrifices. Say what you will about dodging lightning bolts, but at least that was still infinitely more fun that having to farm the same mob for 1 week to upgrade a weapon.
Also semi-related to the above section was just how ridiculous the gil grind was. Having to farm 1 mob, which drops an item worth a mere 150k, with a low drop rate, when you require upwards of 6 million gil. You pretty much have to farm turtles or sacrifices as well, since no other mob can be viably farmed to upgrade higher level items, making the entire process extremely monotonous. Combine this with the fact that equipment had far less of an impact, and you have a terrible system. When you can beat the hardest mobs in the game using no accessories, and starter weapons, something is wrong. If you use anything but top tier items against the hardest enemies in other games, you can expect to a rough ride, while in FF13 it may mean a fight takes a few more minutes to finish.
FFXIII was also lacking when it came to sidequests. Most of the Cie'th stone missions were just higher level versions of what you already fought, and they weren't really new. A decent chunk of them were also repeats (looking at you Zirnita), which seemed like a giant waste of time, and had there not been a trophy, there would be no point in doing the repeated fights. The rewards were also pretty junky considering the effort you need to put in, and although I already mentioned this, having all 64 missions confined only on Gran Pulse was also another bad move. This was also pretty much the only sidequest in the entire game as well, whereas FFXII had Phon Coast hunts, Marks, Rare mobs, optional Espers, bazaar items, and a few minor quests (unlocking the extra areas in the mines, fishing minigame, unlocking Barheim pass, etc), and FFX had Celestial weapons, the Monster Arena, and optional aeons.
As for the story, there were some problems but overall I found it to be pretty decent. Among the main problems though were how annoying some of the characters were in the first half of the game. Probably 99% of the cut scenes with Snow had something along the line of how he has to be a hero for Serah's sake, and again a majority of the cutscenes involving Hope were just him going off about how everything is Snow's fault, along with some ground punching action. They did a good job at fixing this in the later parts of the game though. One thing I really enjoyed was how they paced it and added cutscenes at the end of an area rather than the entire dungeon, it was a good decision to constantly alternate between gameplay and cutscenes. There were a few other problems in the story I had issues with, but they were all slight annoyances rather than major gripes so I won't bother mentioning them.
All I have to say for now, and I probably forgot to mention some things.