...although now you have to justify why 8 is your favorite, haha.
Now that I have got all the praise out the way, let's dig in:
-Fucktarded balance. Easily fixable, of course, but it really dragged the game down before the glory of mods and made it the worst of the best FF.
-Inconsistent level design. Some of it is absolutely great. But others, as we've discussed, could be better. Dollet for example is linear asf on the first visit, but with reasonable purpose as the story dictates. It's fine I guess, as the whole game is not like that. However when you return after the fact, the town opens up to you and yet there is nothing to do. Just that one tavern with the card master dude for the most part. This is decline by series' standards, but luckily most locations do have purpose and good design.
-A super fun story, but with a non-negligible amount of cringe, inconsistencies and not so fun parts (orphanage).
-Interactivity with the environment is toned down. There's still some: occasional puzzle elements and such in dungeons, various kinds of draw points, occassional pickups and frobbables, but it's just less than the other 90s gems.
-Various misc gameplay event criticisms: The chocobo forest mini game sucks hard. The Shaman village sidequest sucks, a lot of stupid running around. Same for the Fishing Master's side quest in FH. Getting Tonberry GF is annoyingly grindy, what it is, 30 tonberrys you have to slay? God bless emu fast forward in this case, that was such a chore. Otherwise, all the many miscellaneous gameplay events are pretty great as per series standard.
-Enc-None ability is decline. Half is reasonable. this ability is often present in other FF but usually right near the end for backtracking/exploration purposes. Here you can get it pretty early, though it is gated by a beastly boss battle I suppose (Diablos). If you're good enough, engaged in the systems enough to be able to beat him, and still decide you hate the combat and want to enc-none, it's probably fair. Also, almost all non-standard command abilities are total shit. Doom? Mad Rush? Devour (it's good but grindy, I always just ignore it). Card mod is cool though. And it's integral you don't get items (or was it xp, I forget) when you use it, otherwise every enemy would get card modded. It would become a no-brainer.
-Once you get the ragnarok, there is plenty to do, but most of it extremely minor. Visit Lunar Tears? Two screens. Visit lunatic pandora lab? Two screens. Visit Sorceress memorial? Two screens. There is of course the two magic islands but they're not that hard because it is the world map where you can save or fly away at any time. they should have been made dungeons. and the deep sea research facility, but that's it for notable locations. Basically, it could be better. But this same thing is an "issue" (it's not really an issue, just could be better) for 9 and 7 too. They all have a fair amount of late game freedom, optional content and exploration, but nothing meets 6's standard here with multiple optional dungeons, redesigned towns as a result of the world of ruin, re-amassing the rest of your party in non-linear fashion etc.
-world map draw points are super vague, or otherwise not frequent enough to teach the player to constantly spam interact, and are never mentioned in the game I think. I wouldn't be surprised if most players played the entire game not knowing there is world map draw points. The magic drawing islands, how are you supposed to know that's its purpose without a guide? MAYBE it is hinted at in-game by NPC dialogue but I don't recall any mention.