I'm not going to repeat my complaints about the combat system or talk about specific bosses or areas, but rather try to express why the game just feels unsatisfying to me. It'd be easy to blame that on the combat or the general linearity in progression, but I don't think that's the case.
The main reason this was difficult to put my finger on is that DS3 is an exceptionally well-made game, by far From's most polished release to date (not having played Bloodborne). Putting aside systemic issues, content quality is very high - level design and aesthetics are excellent, there's a great variety of enemies, bosses at the very minimum all look great and have cool movesets, game is very balanced both in terms of difficulty progression and stat allocation. The world. in spite of being linear to progress through, makes geographical sense, and you can consistently see landmarks from other areas at various points. There's no awful crap like Lost Izalith, no shitty copypaste bosses, no weird discontinuities ala DS2, no clearly broken mechanics. Clearly the game didn't suffer from any development problems (signs of which were very obvious in DS2 for example), and the team made exactly the game they wanted to make. Which, in a way, is even more disappointing, as what I find unsatisfying about it is likely by design, and not by mistake.
For the record, a lot of the tendencies I'll discuss below were already present in DS2, though to a lesser extent. In that way, I actually see DeS and DS1 as results of a particular design philosophy, which the subsequent games started to move away from.
One good word that nicely encapsulates what I dislike about DS3 is "streamlined". In DS1, New Londo, and endgame area, was right under the main hub, and immediately accessible. Ditto for Catacombs, and the graveyard on its own claimed the lives of many a newbie player. Nothing stops a player from wandering into Valley of Drakes from the Darkroot Basin and going into Blighttown from the other direction. The player also is not given any kind of specific goal, and left to wander aimlessly until they find something plot-critical. All of this was significantly toned down in DS2 (which gives you a lot of places to go, but they're all places you can handle, endgame is safely locked away until you're strong enough), and all but removed in DS3 - here, you follow a nice, straightforward path across the game, with a clear and unambiguous goal, with nary a hint of uncertainity or danger. All the obscurity and quirkiness of the early Souls games has at this point be scrubbed away in favor of producing an edgy-but-streamlined combat simulator.
And that's another issue, the extreme focus on combat, and a very narrow version of it, where you enter a large room (so your precious self can have enough space to spam roll), and just fight whatever enemies are in it, rinse and repeat, almost like a series of small arenas. As such, enemies are designed with a focus on being individually challenging, with fast and unpredictable movesets and high damage, and enemy design is almost completely disconnected from area design. This is in contrast to DeS and DS1, where enemies were often not so difficult in and out of themselves, but were an integral part of an area's challenge. Furthermore, while encounters in those games were often easier individually, bonfires (or shortcuts in the case of DeS) were much more sparse, making it more difficult to progress on luck or persistence alone. DS2 and DS3, on the other hand, positively shower the player with bonfires and shortcuts (some of which in DS3 are just straight-up superfluous and silly), which also lets them get away with their more extreme encounter design. It feels like the developers thought that the essence of their formula was this kind of high-lethality combat and streamlined the rest of their game in an attempt to refine this particular feature.
And this brings me to level design. In terms of aesthetics and layout, it's excellent, albeit somewhat formulaic - as if all the level designers adhered rather strictly to an explicit formula, likely a set of design guidelines from the top. So, they do have all the shortcuts and loops we've come to expect and enjoy, and are fun places to explore thoroughly, but lack variety overall. There's very nearly as much variety in DS2 DLCs alone as in the entirety of DS3, in my opinion.
The main issue here, though, is the previously mentioned disconnect between encounter and area design. Recall DS1 for a minute, think about the levels, and just how different they were from each other. Blighttown wasn't just a cool looking place with its own enemy types, it was a confusing structure with treacherous and narrow walkways, danger of falling to one's death at every corner, hidden snipers with toxic darts, and large distances between bonfires. Blighttown isn't challenging because it has enemies which would be difficult to fight in an arena, it's challenging as an environment. Sen's Fortress is challenging because it's a long gauntlet full of traps, not because snake-people are hard to kill (although snake-people can be hard to kill when placed on a narrow walkway amongst swinging blades). Tomb of the Giants isn't challenging because of skeletons, but because of the poor visibility and a layout that takes advantage of it. And so forth.
There is comparatively less of this in DS2 (though it does get props for Shrine of Amana, the foggy forest with invisible enemies, and some smaller touches), and almost none in DS3. DS1, although its combat is relatively easy and exploitable, still managed to maintain a feeling of danger by constantly putting the player outside of their comfort zone and introducing new elements in its level design on a regular basis. I do feel like this is the main reason why exploration in general worked so well in that game, and is yet unmatched by its sequels in this respect.
If you think I'm exaggerating or cherrypicking to support my argument, join me in a little experiment. One of the notable things about Souls games as they were introduced, was the ease of falling to one's death. In an era of handholdy corridor shooters, this stood out a lot, and if you watch someone completely new to the series play, chances are you'll see them die this way many a time. Level design in DeS and DS1 makes great use of this fact. Now, think back to your DS3 playthrough, and how many times you were ever in danger of falling to your death. Not so many, right? Even this kind of simple element was scaled back to make room for more convenient combat. You can also think about how many boss rooms there are in DS3 which are not huge, empty arenas with no additional features, while you're at it, and you may start to see my point.
Unfortunately, given the likely commercial success of DS3, I expect that we'll get to see more of this in future From games. Ironically, despite the continuing focus on punishing difficulty, the series is slowly being dumbed down for the mass market in its own peculiar way.