I still haven't actually finished Sun & Moon. I think I'm currently in some library, looking up something about some legendary pokemon for reasons I can't remember. It makes for a strange contrast to the very similar X&Y and ORAS, both of which I just loved, and blitzed through in roughly two weeks, deliberately slowing myself down to make them last.
For me, the issue wasn't the new pokemon, or the available older Gen pokemon, or replacing gym badges with trials, or the difficulty level. All of those things seemed absolutely fine to me. What killed Sun & Moon for me was just how intrusive the story was.
I don't mind that the story wasn't very good. The vast majority of Pokemon games have pretty poor stories, but it doesn't matter. That's not what I play Pokemon games for. I don't care about the evil Team's plans, or the legendary being woken up this time, or my rival's personal journey. I just want to catch Pokemon and make them battle other Pokemon.
Sun & Moon's story wasn't any better or worse than any other Pokemon game's. But it was much harder to ignore. It seemed like the game was constantly stopping for yet another cutscene, and they were so pointless. Just Hau or Lillie or some other person rambling on about nothing for an eternity. To rub salt in the wound, they give you Mass Effect-style dialogue options - none of which make the slightest of difference in how the story progresses, meaning that all it achieves is slowing down the cut-scenes even further!
It probably wouldn't have been quite so irritating in earlier generations. Mostly because the cut-scenes were much simpler and progressed more quickly. NPC enters from offscreen, talks to you for a bit, you can hold down A to make the dialogue appear on screen more quickly, and then its over. B&W's storyline could be pretty frustratingly intrusive too (I could quite happily go my entire life without reading another of N's banal monologues ever again), but didn't suffer so much from this because the dialogue could be skipped through more quickly.
But now there are establishing shots and camera angles, and the NPCs can actually gesture and do things beyond walk and stay still, you get to see the protagonist staring and grinning like has suffered brain damage in response to all of this, and in theory this should make these cutscenes more interesting, but in practice it just makes them longer. On top of that, the dialogue doesn't seem to be able to be sped through in the same way, instead the speech bubble appears, the dialogue appears, then it skips to the next line in its own time. (I could be misremembering here, it's been a while since I last fired up Sun & Moon.) I don't care about whatever the heck is threatening the world this time - just let me battle Pokemon!
It wouldn't have been so bad if the story - like in most Pokemon games - didn't take itself so damn seriously all the time. I liked, for example, how deliberately stupid Team Skull was, and that fire (?) trial made me genuinely laugh out loud in a way I rarely do while while playing Pokemon (those post-game scenes in ORAS where you get the extra starters were similarly uncharacteristically hilarious, to the point where it seems like you've just wandered into a different game.) Were the tone a bit more like this, and less aggressively dull, I wouldn't have nearly as much issue which how much prominence the story took up in the game.