We know enough to say that Tabata's version of the game is way different from what Nomura had originally planned. Stella, who was an equal to Noctis and had the same powers as him, got replaced by Luna, who was poorly developed and died so quickly that you had no time to develop an emotional attachment to her. The Emperor of Niflheim and Verstael, the Chief Scientist of Niflheim, were originally intended to be main villains in Nomura's version, but in Tabata's version they barely show up and both die off camera. Ignis was supposed to be Noctis' military strategist in Nomura's version, but ended up basically being Noctis' butler in Tabata's version, British accent and all. The planned Insmonia invasion and subsequent escape from Insomnia by Noctis and his friends was dropped from the game entirely in favor of Noctis and his friend's already be gone on a roadtrip when the invasion happened. We know other details from a leak, but I won't get into those here since they aren't confirmed. We also have a good idea of what Nomura had planned for the combat system, based on that 2011 trailer and from what we've seen of Kingdom Hearts 3, which is way different from the hold button to attack system that Tabata came up with. In my opinion at least, Tabata did botch the game, though I don't think he is entirely to blame, it is clear that Square rushed him because they got sick of waiting for the game, he basically rebuilt the game from the ground up in 3 years. Square should have let him co-direct the game with Nomura, with Nomura's main focus being on KH3 with assistance provided to Tabata as necessary, that way development wouldn't have needed to be restarted and we would have got to see more of Nomura's original vision for the game.
Except those rumors were just well, rumors. This was never the vision Nomura had. All along, Stella was put into the same role as Luna EXCEPT on the very first trailers, when there was no plot AT ALL (and are the ones that feature a fighting Stella). The only parts that changed vastly were the pacing and the invasion of Insomnia (in fact, this was where Tabata and co hit a snag. They wouldn't be able to deliver by the end of 2016 if they tried to keep it in.
People need to stop this "Tabata ruined Nomura's game" because there was no such thing as "Nomura's game". It was just a collection of trailers and concepts similar to Agni (which I hope is the project they are working on at Luminous Studio, since they mentioned a new AAA IP). There was no actual plot beyond "Noct and Stella are Romeo and Juliet and fight a so called dictatorship" until you realize there was no plot in between. Admittedly, the concept would've been better than a relapse back into "turning Eos into Remnant II" but hey, at least we got a bloody game and not a load of warm air.