There are a few things that stand out as the main things ruining it atm. First and foremost, enemies randomly spawn continuously around you. If you stand still for less than a full second they can spawn in even. And they'll appear behind you even if there is a wall or cliff or something in the way. They constantly hound you and it just is not a fun thing. This is absolutely fixable via modding, but I think it is also something fixable via cheats. The next big thing is item durability is insanely low for everything. You're supposed to live mostly on what you can cobble together as you delve into the Abyss and the biggest reason that is non-viable is the aforementioned random spawns constantly hounding (people seem to have the idea you're supposed to just stay constantly running and avoiding everything, but you can't really do that when the game requires you to eat food from time to time but you're surrounded by things that won't let you eat...) If the first thing is fixed it would make that a lot better, but it still is not fun how fast everything breaks and a cheat to give items greater durability would be incredibly nice to have. (Probably an unbreakable option would also be nice, though for many of us it is sufficient simply to decrease durability drain.) And finally, I personally would like to see the sensitivity of the curse effects decreased. It begins to show stuff on the screen well before 10m and pretty much the instant you hit 10m gross ascension (btw it ignores net -- eg going down doesn't count, so if you walk across a bumpy ground then it counts as ascending even though the bumps go down too) it hits really hard to the point that your character starts vomiting and such (which costs you food, makes you stop, and typically small enemies are all too happy to show up to take a nick or two on you just to be annoying.)
These things are fixable. Whether it's enough to make the game truly good or not is a different discussion. There is a lot I do like about it and if those things (particularly the random spawns more than anything else) could be improved it would really help enormously. There are a few more fundamental things that aren't likely to change too much even with modding though. The areas are (mostly) not large open areas but instead are relatively small "maps" connected together by area transitions. Actual exploration is a lot more limit because of its general design choices, but at the same time I wouldn't call it completely non-existent either. Also, it has a rather simplistic graphics engine with relatively low detail models, textures, etc etc. Maybe that can be improved with heavy modding, but that is likely dependent on the game being popular enough for long enough for people to actually do so. A lot of people liken it to a PS2 era game graphically and to some extent that is sort of true, though obviously it's much higher resolution and generally overall better than a PS2 could even theoretically do (let's call it an early PS3 game in this respect then I guess.) Overall this isn't that big of a deal, but then there are times where it can be a hindrance such as when you run across some of the larger creatures that are supposed to be positively terrifying and they end up looking a bit more simplistic than they ideally should. Still, the graphics aren't actually bad, just not what you expect from a game with a AAA pricing scheme. Actually, now that I've said it, this game feels kind of a bit like an Ys game with survival elements and clunkier combat, lol.
Anyway, I hope that sufficiently answers your question. I do think the main thing though is you definitely would want to wait for the price to come down. No one is happy with the price for this game given what it delivers. At such time as its price drops considerably to, say, half or so it will be much more in-line with what the game actually is.