Ignore the morons in this thread, the ACTUAL reason people don't like DS2 is because every aspect of it is either clunky or not well realised, despite it having some good ideas behind it.
ADP is a good idea on paper, but in practice it essentially becomes a mandatory roll stat. With the animations not changing between levels to a noticeable degree, having low ADP essentially results in your rolls LOOKING like they should work, when they in fact don't (I can't remember off the top of my head, but I think low ADP literally has 1 iframe). Rolls that would previously work in DS1 now don't because of the shortened iframes. Given the already questionable hitboxes of the Dark Souls series, this compounds into a frustrating mess of confusion after getting hit where you're not quite sure if your ADP was too low, or the game just flat out broke.
Light management and torches was supposed to be a good idea too, but the game is so bright there's literally no reason to ever carry a torch. SotFS somewhat fixed this by adding some genuinely dark areas, however the mechanics essentially become "light sconce, put torch away". When there's no enemies nearby, this mechanic is pointless, when there are enemies nearby, this mechanic is tedious and annoying. The Torch being on a timer is supposed to add resource management, but generally the timers are really high to the point where you don't have to worry. If you end up playing badly through a section and DO run out of time, you're doubly fucked because now you won't be able to see, so have fun farming The Gutter to find more.
Lockstones and Fragrant Branches are also a good idea on paper, since more resource expenditure can be really interesting. Unfortunately, half the lockstones are completely useless trolls, or are MP focused (like dimming the lights in a dungeon, which can actually HURT you in singleplayer), and you won't know which one does what, and since you can't reload a previous save due to the design of the bonfire system, a limited resource is now wasted because you got unlucky. Fragrant Branches aren't as bad, almost all of them always lead to something good, however there is one fragrant branch that leads to another fragrant branch, and there's only enough branches in the game to unlock every statue, meaning if you do them in the wrong order you will be locked out of one of them through no fault of your own. Again, good idea, horrendous implementation.
Lifegems are a nice alternative to estus, giving slower life regeneration that's designed to be used outside of combat. And yet they provide more mobility during use than the estus and a faster animation (for some reason), and can be farmed or bought in infinite quantities, resulting in you being able to carry 99 of them at any given time. This was supposed to augment the healing system in an interesting way, and instead just ends up completely undermining it, to the point where you should be able to heal through every fight. The regen ring is also findable very early, meaning that healing between fights is trivial and any sort of long-term health management goes completely out the window. One of the best aspects of DS1 was how limited estus was, making long runs (like the first trip through Undead Burg past the Taurus Demon) extremely challenging for first-timers because few mistakes were permitted. Humanity somewhat undermines this and is farmable infinitely, but outside of the DLC still remains relatively rare for most of the game. In DS2 you can buy lifegems in infinite quantities after completing the first area. DS3 is probably the best here, as it's healing is inherently limited and can't be augmented with farming at all, since Ember only works once per life, leaving you with Estus and the occasional Divine Blessing for healing, which gives much of the same feeling of tension as your mistakes matter as much as they do in DS1.
DS2 also has a lot of design which I believe actively trolls the player, in a really bad way. There are many instant-death pits that look barely different from the normal terrain (especially in the dark, The Gutter is notorious for this), bosses crammed into absolutely tiny rooms to make them as annoying to fight as possible, "spambushes" where otherwise normal looking rooms have upwards of 10-15 regular enemies crammed into them, which all attack you at once (often from behind), floors that give way to lava, killing you instantly, and numerous other traps like doors that lock behind you. DS1 obviously had these too, but most were limited to Sens Fortress (which was inherently designed as a trap-laden labyrinth), and were generally rare outside of that location. Probably the most egregious sin of DS2's design is hiding the main blacksmith behind a semi-secret area that's easy to miss, which just completely fucks over new players entirely.
DS2 had the potential to be significantly better than DS1, as it tried a lot of new ideas and made genuine attempts to improve the gameplay in interesting ways, like making Pyromancy actually scale with player stats rather than being based on how wealthy you are. and trying to undermine the roll-supremacy in DS1. It's actual implementation, however, was extremely poorly done, and while SotFS fixed some things, it also made a lot of things significantly worse. It was supposed to be an interesting twist on the DS formula, and instead it's a trolly mess where barely anything works how it should and is an absolute chore to play.
I can see why people like DS3. It took some of the better aspects of DS2 and refined them (power stancing became dual-wield weapons, pyromancy scales based on faith and int, respeccing from items instead became linked to an NPC, etc), while also being smooth as butter to play and not filled with bullshit to the same degree as DS2. I can also see why people consider it extremely boring, as it's basically peak Dark Souls mobility and combat at the expense of everything else - poise is gone, as is pretty much any interesting niche-build, it might as well be a boss rush game (the bosses ARE very very good, though).
The TL;DR is, don't play DS2. It's not worth it. If you want an interesting game with lots of build diversity that doesn't hold your hand without being total bullshit, play DS1. If you want a fun Soulslike with really good combat and smooth gameplay, play DS3. Just don't ever play DS2 for any reason, other than completionism or to explore it from a game-analysis perspective.