DAI was possibly not a slog for me as I think it was actually my first open world experience on that level, and I had never played MMO's either (I know a lot of people say DAI is very MMO-ish) It felt very new to me in a lot of ways. I also really enjoyed the combat in DAI so that kept things fresh for a long time. Even if I was doing a simple fetch quest, I was having a blast because I loved the combat, the game was beautiful, and I felt attached to the characters. I would agree about the gated, "power" based story progression not being a thing I liked in any way.
I totally agree w/ prag16 that DA:I and ME:A suffer to a large degree from the same problems with open-world bloat and underwritten or poorly presented questlines.
I'll float a couple thoughts, though, on why DA:I might feel a little less like a chore for some players. First, travel between zones is faster and easier, and quests are usually contained to one big region. ME:A made me go "fuck, back to (e.g.) Kadara, again?" quite a few times, and that required travel back to the ship, then out of the system, then into the other system, then into orbit, then onto the planet, then into the slums, and then finally into the zone I needed to be in. In DA:I I'd just fast travel and sit through the loading screen, and it was a rare quest that required me to hit multiple areas in the first place.
This issue gets worse later in the game when your quest density starts to decline, making it harder to efficiently chop off many quests simultaneously in one place.
Also, while DA:I's decision to gate story points behind "power" or whatever it was called was indeed stupid, it was in my playthrough(s) nearly irrelevant - I found that it was very, very easy to hit the thresholds necessary to advance the story, should I wish to.
Lastly, and much more subjectively, I found DA:I's environments somewhat more varied and interesting; others may disagree. I didn't love DA:I, though, for all the normal reasons. I'd rate it above ME:A, but not by very much, and its filler was indisputably more underwitten than ME:A's. They essentially didn't even try to tell a story in a lot of quests.