As someone who got through ARR not that long ago for the first time and is in the middle of HW right now:
The main issue that I have with ARR is it doesn't have much in the way of compelling character or plot beats to actually make you want to see what happens next for quite a long time. That's not to say it has none (I liked the refugee plotline idea in Ul'Dah for example), but it's usually spaced very far apart because of the poor pacing of the story. Even when you are doing setup for a slow burn type story, it should still be interesting the whole way which I don't think ARR does that good job of.
To give two big examples of what I'm talking about, let's talk about the Scions and the villains.
The Scions, I only found a reason to care about Y'shtola, Thancred and Alphinaud because they were the only ones that felt like they had a personality to speak of. It wasn't the most complex or nuanced characterization, but it was SOMETHING. This isn't to say that Papalymo, Yda and Urianger don't have personalities but they are not especially evident until like waaaaaaaaay later after you are past ARR, throughout the entirety of ARR I just did not care about them at all. Minphillia especially is quite bland and disappointing as a leader character, in that she barely had any real personality and felt like she existed solely to send you on fetch quests and make you go through two loading screens every time. I know this likely involves 1.0 backstory stuff I don't know about, but just from my limited perspective, it felt like you could have excised her character completely and just had Alphinaud be leader and nothing of value would be lost, and the story would be more streamlined.
The villains, for the vast majority of the game the villains you have to deal with are the Ascians (more specifically Lahabrea) and for the most part I found them lacking. I understand that being enigmatic and setting up a mystery is part of the intention there and that isn't a bad thing inherently, but I feel in ARR the execution of this isn't super good. I just never felt like I knew anything that would make me care about them or what their deal was, I wasn't really invested them in at all. Gaius and his entourage may not be THE best villains or anything, but I started to enjoy the story more when they came into focus because I knew things about them, their motivations, the relationships they had with each other and the rest of the cast, which made it easier for me to care about them. Unfortunately you only really start seeing more of them near the end of ARR.
And this is all before I get into another big issue, which is the very middling voice direction of ARR with the old actors. Is it like the worst VA ever, no, but it isn't particularly good either, it's equivalent to any mediocre anime dub, which makes it hard to really care about the cast since most of them sound so bland. I honestly did not realize Urianger's thing was being a stoic emotionless guy until Moenbryde was introduced to highlight that fact. Minphillia's VA is also pretty bad and was another thing that made it hard for me to care about her.
The main issue above all of this is simply the fact that ARR takes a considerable amount of time to get through. I'm not sure how long it is exactly, most estimates say it's around 30-40 hours, which is a lot of time to ask for something to become compelling.
I respect the fact that ARR had something substantial to offer people and saved the game, otherwise it wouldn't have continued and received all these expacs, and I wouldn't have continued playing it if I wasn't enjoying it on some level, but from my perspective ARR has big issues as a story and there is a good reason why people criticize it as being a slog.