About the recent influx of negativity about the game:
I have a feeling it has to do with the fact that the later stages of the game - especially Skellig - does, in fact, have weaker writing & quest design on average than Velen/Novigrad.
I was personally engaged the whole way through Velen. The main area arc - Baron quest, along with the Crones quest attached to it, were well designed, heartfelt, and expertly written. There was never a dull moment, so to speak, especially because this was at the beginning of the game just as you were learning about the world, getting used to contracts, etc. It was all new, all fresh, and left a huge positive impression that I think was responsible for all the raving in the first 50 pages.
Then you got to Novigrad, which was a nice change of scenery, and while its quests were slighty less well designed, you got to interact with the old crew ie Triss, Dandelion, Zolten, Djikstra, etc. The characters & their intrigue kept the pace up and while the main area arc wasn't as memorable as the ones in Velen, the extra interactions you got with old friends kept the experience together. The awkward unrequited love angle with Triss was also well done, and ought to be an example for how to do romantic content correctly in games. Up to here, the pacing was still on par, and I think the city setting & the political intrigues in Novigrad were a healthy dose for a developing story.
But then you got to Skellig, and this segment is very obviously the weakest in the game. The interactions with Yennefer were decent, but outside of those, Skellig is underdeveloped, has mediocre writing, and is full of fantasy cliches and boring characters. There was basically no twist in the main area arc, and the whole competition for who gets to be the next king plot was filled with fantasy cliches. The side-quests were also very mediocre - I especially deplored that sword fetch quest which reminded me of Dragon Age Inquisition fetch quests where you had to travel to different spots of the world with barely any pay off. There were only a few memorable quests in all of Skellig, and the rest is basically filler.
I'm pretty sure Skellig was not written by the same people that wrote Velen, otherwise they were so pressed for time they decided to just rush it, because the quality drop is obvious and I struggled to get through the area, while in the first few days I wasn't able to stop myself from playing. This comes at a bad time in the game, because Skellig is around the midway mark of the game and it's around this time that the newness wears off and players begin to get bored. They needed a motivation booster around this time of the game, and Skellig was not it.
Reading other people's posts, it looks as though the game picks up the pace after Skellig, but judging by the dearth of buzz around the end acts, I've a feeling it also wasn't up to par with Velen, which explains the increase in negativity as people start to finish the game.