I've been playing dwarf fortress on and off for years. I've modded the game, delved deep into all those notepad documents, mostly to try and figure out why things happen or try and improve them. That said, it wasn't until I tried to get someone else into the game recently that I remembered how esoteric most of the game actually is. Complexity isn't innately bad, but a lot of things in dwarf fortress have been piled on top of other things so many times that everything is a mess.
The biggest source of frustration is that literally nothing is clear on its own. Things that should make sense are impossibly complex for no logical reason. Examples of this include: military organization, bin use, gathering discarded items, minecarts, stockpiles, work orders.
The military is a mess. Trying to figure out how to make archers work correctly is like pulling teeth. Trying to figure out just how to make the military function as intended is equally bad. Most of it is because you have to go through menu after menu just to set basic things like schedules and equipment. Ammo is extremely difficult to work with, and it's never clear why your dwarves aren't doing the things you want them to do.
Bins. I have no idea why bins are such a pain, but they are. Mostly because only one task can be used for one bin at a time. Meaning that if you have any sizable amount of material, large amounts of materials are considered unusable or gone because they're being accessed by some other dwarf in some other part of the fort. Oh, you need cloth, because you need multiple clothiers? Sorry, all that cloth is considered gone, because one dwarf at a different clothier's shop has taken up the entire bin. Maybe, your guys can't get new clothes, because someone has decided to stud something in that bin. Maybe you can't get weapons because a miner has decided to drop his pick off. As it stands, you either need massive amounts of land in order to provide for all this stockpile space, or you need to suffer under the bin bug.
Gathering discarded items is awful. Have a major siege? Now you have to go through and manually forbid or allow all the items out there, otherwise your dwarves will either haul nothing or all of them. If you have hundreds of items out there, then you'll be busy for a very, very long time. it's insanity. How is there not a 'forbid/allow all' command when selecting things on the ground?
Minecarts are a mess. Just making them function, or figuring out their function, probably takes a mathmatics degree. They seem to only function if the stars are aligned correctly; I have a better chance of summoning an elder god than I do at making a mine cart serve it's purpose.
Stockpiles are a mess. More accurately, getting dwarves to only take certain things from stockpiles requires so much micromanagement that it becomes a chore. You practically need to double your stockpiles, otherwise dwarves shove everything together into 'finished goods.' Dwarves will decide to continually encrust socks over and over rather than any of your actual trade goods, because they can't decide what to do. Eventually, you're just making stockpiles for every item because you can't keep track of who is grabbing what at what time, and at that point you lose track of where everything is.
Work orders are a pain to deal with, and there are a lot of bugs. For example, sometimes your work orders just don't work at all for seemingly no reason, or go on forever. Tell your dwarves to do something like encrust or stud, and they'll literally never say that they've completed anything.
At this point, I think it's clear that the main problem are all the bugs. Look, I love new content. I do. But I would love nothing more for dwarf fortress, and nothing would make it better, than if instead of adding new content, the huge backlog of bugs was dealt with, and everything was streamlined. Almost nothing makes sense at this point in the UI; the orders tab isn't clear for example and it's not entirely certain what different things do or don't do. Burrows are a mess to deal with, because you have to attach literally everyone into them if you want them to work, which is a pain for a fort with more than one hundred people. Seeds disappear completely without being edited in the raws, which is a problem because if you have a fort growing lots of plants, you'll quickly run out of seeds due to them being disappeared by the game.
Most of my time playing dwarf fortress isn't me going 'man I want to try doing this!' it's me playing until something goes wrong, at which point I have to play detective and search through wikis and raws to try and fix the issues that haven't been fixed in the game. That's what breaks things most for new players, imo. They play the game, hit something like the bins not working, and someone goes 'okay use a quantum stockpile' and upon seeing how difficult that is to do, quickly figure out that it's probably not worth doing.
I can say that, as a veteran player, the thing that stops me from making more forts and going back to the game is just thinking about all the micromanaging and messing around with the stuff I've mentioned. Just trying to make things work as intended is a mess. There's so much STUFF in the game, but so little of it seems to work coherently together. So much needs to be streamlined and cleaned up and put together in a clear and concise way. Because right now, players are as much bugfixers as the actual programmers, with how many workarounds are needed to make the game work in any sort of stable way.
Essentially, the biggest impediment to people playing dwarf fortress is the actual gameplay of dwarf fortress. So much has been built atop unstable foundations and swept under the rug and put off that at this point, new players only see it the way you look at a mess of tangled wires behind a desk. Yeah, you could try to figure out what everything does, but you probably won't. Which is a shame, because there's a lot of great stuff with dwarf fortress. But if the goal is to open it up to new players, then the game needs to clean up all the bugs and streamline the actual gameplay to a point where it makes sense and you don't have to search through wikis and forum posts to make the game work as intended.
Or, to put it another way: if you have to rely almost entirely on third party mods and scripts and applications to make the game at all useable, then much of the game isn't actually useable. If players are relying on dfhack and dwarf therapist to use the game, because the game itself makes such things difficult if not impossible, then that's a problem with the game. Many, many quality of life upgrades need to be done with the game to make it accessible to anyone who isn't interested in a college course amount of work just to play it.