Oh, this game was so good! It set such a high standard for me that no subsequent Final Fantasy game has been able to meet. I loved it so much that Final Fantasy 7 turned out to be a huge disappointment to me by contrast. Obviously, this is all my opinion and may be reflected by no one else in the world, but as a comparison between 6 and 7...
1) The characters in 6 were so much more fun to play. Each was completely unique, with different skills and abilities. Edgar could use tools. Sabin could blitz. Mog could dance. Gogo could copy everybody! In 7, I felt like virtually all the characters were interchangeable in combat (with the exception of Aeris being more specialized in magic than the rest). Abilities/magic were driven mostly by materia, which you could swap around and equip on anybody as needed, making everybody essentially the same.
2) The story in 7 was a confusing mess to follow. Some blame it on a poor translation, and perhaps that's what it was. But after my first playthrough, I recall not really understanding Cloud's past or his relation to Sephiroth; the flashbacks were janky and weird. It's been so long since then that I can't recall specifics on exactly what I had trouble with, but I know years later I ended up reading an online summary. Only then did I actually understand the nuances of the story correctly. By comparison, the story in 6 was straightforward, in a good way.
3) Minor gripe: I was disappointed that 7 only let you control three characters in combat rather than the four characters in 6. I felt like that was a step backwards. But going back to my point #1, it didn't matter as much because all the characters played the same anyway.
4) In a Final Fantasy game, I prefer a fantasy setting. FF6 gave me that, as had every Final Fantasy game I had played before that. Sure, there were steampunk elements to 6, but it mostly felt like a fantasy game setting. FF7 ramped up the tech elements and updated the setting to something more modern, something that usually felt more sci-fi to me than fantasy. As a result, I was just less interested in the game world overall. Don't get me wrong, I love science fiction, but it's just not what I want in a Final Fantasy game. Final Fantasy 8 was even worse in that respect, but that's a discussion for another day.
In a way I pity FF7. Basically, FF6 ruined it for me. And that shouldn't be its fault.