Fanservice can be fun... for a little while.
I bought this game probably for the same reason you did: nostalgia. I grew up on the Final Fantasy series and being able to pit Terra and Squall or Zidane and Kefka against each other in a ring is just an awesomely simple and satisfying idea... but while Super Smash Brothers did this flawlessly, Dissidia falls short on several fronts. Firstly, the gameplay is repetitive despite the fact that each character has a diverse moveset. Some of the moves are really cool, but that's not enough to look past the awkward controls. You have to hold forward and circle/square or back and circle/square to execute different moves and you can only use the thumbsitck for this, which is awkward at best. I found that my character would use moves that I didn't want him to use often, and sometimes he would just use moves for no reason because the controls are too cluttered. It's way too easy to use moves accidentally or at the wrong time, and that can lead to frustration when battling opponents because timing is everything in this game. The battles themselves are confusing at first and once I understood how the mechanics worked I found myself wanting desperately for them to be more simplified. Firstly, both you and your opponent have an HP bar and a bravery meter. You need to steal your opponents bravery with circle moves, the more bravery you have the more HP damage you'll do to the opponent when you use your square moves. I thought this system was overly complex and just added a level of complexity to the game that wasn't needed. Why not just skip the middle man, take out the bravery meter, and make everything do actual HP damage... that makes more sense, doesn't it? But that's not the only thing Square Enix tries to make overly complex in this game.
Story mode is just a pile of nonsense. It's way too complex for the simple game that Dissidia should've been. There's so many ideas trying to be executed in story mode that it just becomes a clustered mess. It's hard to explain each and every mechanic being used here. First of all, there's a world map... why? Why is there a world map? All the action takes place in a separate board game map, so why is there a world map? Just like the bravery meter the world map serves as an unnecessary middleman between me and the action. It's just another game mechanic that Square Enix added to try to make the game seem more complex and longer than it actually is. The first Dissidia game was solely board game maps, and that worked for what it tries to do. There was no need for a world map, so why did they feel like it was necessary to add one in this game? If that's not enough, after the actual battles you gain EXP and collect items and accessories that you can equip. I didn't realize I was playing an RPG... I thought I bought a fighting game with Final Fantasy characters? Leveling up in story mode seems like an OK idea, but why equipment and accessories? This is the type of game that I want to be able to pick up and play without having to worry about my equipment and character level... and the game does make you worry about these things because the only difficulty between the different enemies in this game are there levels, there's no skill here... whoever's the highest level wins, pretty much. You can be breezing through enemies, thinking you're doing well, then the boss of a particular game board pops up all the sudden at level 19 when you're only level 10. That's nonsense to me, this is a fighting game... why do I have to grind my guys in a fighting game? Shouldn't this be about skill and how well I can control my character?
Now I'll get into some lesser annoyances: The fact that this game even has a storyline is unnecessary to begin with. Sure, it has some pretty cutscenes mixed in as you progress in the story, but the characters and the actual plot are so nonsensical that I find myself skipping the cutscenes more often than not. The voice overs and the personalties of my favorite Final Fantasy characters were butchered in the storyline to the point where it almost made me not like some of them as much as I used to. It's hard to watch Zidane, Lightning, Squall, Cloud, and many others take the situation they're in so seriously and not laugh a few times. I felt like it dumbed down the actual characters and made them unintentional parodies of themselves. Like I said earlier, I skipped over most of the cutscenes after watching a few of them just because they were so awful. This game should not have had a storyline to begin with: it's a fighting game in the vain of Super Smash Brothers... it's supposed to be light hearted and not take itself too seriously. Super Smash Brothers Brawl had CGI cutscenes too, but they were all amusing fight scenes that were actually fun to watch. Dissidia has CGI cutscenes that try to be as emotional as the mothership Final Fantasy games and take themselves way too seriously. This doesn't really take away from what gameplay there is but I certainly could have done without all the cutscenes and pseudo character development. I understand that to have a storymode there needs to be an actual story going on but it certainly didn't have to try so hard to take itself so seriously or even make sense to begin with: like I said before, this is a fighting game at heart: it should be about gameplay. Maybe they could have gotten away with this if the story wasn't so ridiculous to begin with, but who knows.
Overall, the battles themselves can be fun but they get repetitive and stale after a while. The storyline sucks, the world map sucks, the menus suck, among other things. The biggest problem with this game is it's unnecessary complexity which is evident in every single aspect of the game.