Well, the good news is (assuming we get you strapped back on the wagon) we'll have two games before a new console reminds you of all the other great games from the era you could be playing.
...although I never played or watched dissidia. It was the first 'lets put FF into a blender' game long before FF14, and released sometime after the biggest spike of 'lets write fanfiction where all the FF characters go to school and are taught by other FF characters' that I recall.
What's it like?
I have barely touched the Story mode because I want to avoid spoilers for games I haven't played yet (though judging by the way it seems like every character was plucked from their timeline and have fuzzy or no memory of their original lives, I probably needn't worry about that), so I can't speak for the narrative, but gameplay wise:
Dissidia is a 3D fighting game using a unique 'Bravery' system. Each combattant has a Bravery gauge, and basic attacks drain the enemy's Bravery to add it to your own. Bravery is separate from HP, which can only be depleted by HP attacks; an HP attack deals damage equal to your Bravery score. So opponents are fighting to drain and stock Bravery before expending it in big HP attacks that actually wear down the enemy's health, with Bravery reseting after an HP attack so it must be gained again and puts the character who just attacked in a position of weakness. (Exalted players will immediately recognize this as Literally The Ex3 Combat System).
The roster of playable characters was very simple in the original release: Each game in the series is represented by its lead protagonist and its most significant villain. That means Cloud and Sephiroth for VII, but it also means Cecil and Golbez (rather than Zeromus) for IV and an aggregate 'Onion Knight' character vs the Cloud of Darkness (rather than Xande) for III, which gives an idea of which antagonists were considered huge duds in their own games. Each character has its own unique fighting style based on some trait from the game they were for; in Cloud or Squall's case it's very obvious (gunblade/buster sword, Limit Breaks directly borrowed), but in the case of older games it's lead to interesting interpretations - for instance, Firion is a 'weapon user' fighting with a variety of weapons, to represent II's particular skill system based on individual weapon classes. Some characters are better at Bravery attacks, some at HP attacks; some are better at range, some close-up. Terra's Bravery attacks, for instance, fire homing spells like Fire or Blizzard that can hit powerfully and at range, but her moves are low-priority so an enemy up close can hit her through her start-up animation, so you want to ideally be away from the enemy. Her HP attacks are huge spells like Flood or Tornado, with the latter surrounding her with three individual tornados that circle the screen while she chants, forcing the enemy to either stay far away or risk getting hit while approaching her to break her out of the attack.
Each character has an EX Gauge. When it's maxed out, they can enter EX Mode, a transformation that grants a number of benefits like passive regeneration and modifying their attacks. If you successfully hit an enemy with a HP attack while in EX Mode, you will cause a special finishing move called EX Burst, requiring you to enter special inputs to maximize damage and tremendously hurting your opponent. EX Modes and EX Bursts are based on iconic Limit Breaks, boss phase transitions, or similar things: Terra, for instance, enters Trance and turns into her Esper form, allowing her to fly freely, and her EX Burst is Riot Blade, based on her desperation attack from FFVI that we never saw in this Let's Play. Ultimecia, meanwhile, Junctions Griever, and her final attack involves Time Stop, which is cool.
It's really, really fun to just take any of these characters that we previously only saw in turn-based/ATB systems, a gameplay that's very stiff and obviously more symbolic of the actual battle happening in the story than it is literal, and getting to play them as superpowered fighters zooming around the battlefield and actually hitting things with their swords. Like, I can play Squall and actually get to jump around and hit things with the gunblade and see them fly away, it feels
real in a way RPG mechanics never quite do (though of course I love them for different reasons; I am much more of an RPG player than a fighting game player). And of course, getting to actually play the villains is tremendous fun; I absolutely suck ass at using Ultimecia's toolkit but I'm going to keep at it anyway because being the Evil Sorceress and raining death spells down on foes while doing evil little twirls is just incredibly fun.
The game doesn't necessarily explain itself very well, though - it never taught me that there was an actual Block function, just to Dodge enemy attacks, which isn't always practical. I get the strong impression that I don't fully understand the mechanics, both in combat and out of combat - there's actually a leveling system where you can unlock new moves for player characters, which is
weird, and it's hard to find guides that explain things to you as a total newbie as opposed to assuming pre-established knowledge.
One thing I've remarked is that all characters have a ground toolkit and an aerial toolkit, so Terra casts Fire while on the ground and Blizzard while in the air. However, I've never actually
seen her cast Fire, because even though the characters don't technically 'fly' (they fall if you don't keep them in the air through repeated actions), aerial action is
so easy that it's incredibly likely to just spend an entire fight while in the air. It actually takes effort to bring a character down so you can leverage their ground moves; the default is for everyone to jump in the air and then stay there chasing each other with dashes and double jumps for the rest of the fight, which is a weird experience!
If anything, though, that makes this game a pretty faithful video game rendition of the final fight in
Advent Children.