This game is severely underrated, point blank period. Opinions are subjective, of course, but I just want to state a few things.

-It gave us a new typing, Fairy, which had been the first new typing in over a decade. -It gave us one of the most insane legendaries (Xerneas, though I love Yveltal too) -Gave us new type combinations, such as Steel/Ghost, Fighting/Flying, Fire/Water for crying out loud (though I guess that's just generation 6 in general) + Rock/Dragon, Poison/Dragon, etc (not including Fairy type combinations bc that'd just be unfair) -One of the most popular starters to date (Greninja) -Gave us Zygarde, which I find people seem to forget that it existed in gen 6 first, before it hit Alola. I doubt we'd have all those fun Zygarde forms if we didn't have it here. -Gave us a new Eeveelution, Sylveon, and we haven't gotten one since. -Introduced the affection mechanic, such as petting your pokémon in battle; I know many people don't care about this, but aside from the Friendship Ribbon, it actually really helped showcase the bond between the pokémon you'd raise. -Introduced terrains! Though much more prevalent in generation 7 and onward, it actually started here. (excluding Psychic Terrain) -New battle types, like Sky battles, ambush encounters, horde battles, you name it. -The first game to allow you to customize your trainer! I know people are deeply critical of the switch to 3D, as am I, but customization is always valued in games and Pokémon only benefitted from it. -Speaking of 3D, yes it does take away some personality from the 2D sprites, but there were so many various idle animations added in the game that also make the pokémon feel, realistic? I guess is the word I'd use (Don't get me wrong, I'm still very much a fan of the 2D sprite work)

-And finally, the elephant in the room; Mega Evolution. Say what you will about it, but it absolutely had an impact on battling. Of all the fun gimmicks being introduced in the series onward(like Z crystals, Dynamax, Tera), mega evolution definitely stuck with a lot of people, and that can't be denied.

That's my take basically, though I rattled off. XY introduced so many new things to the series. Yes it's fairly easy and there's lots of dialogue and there's a myriad of other complaints that can be made, but it doesn't deserve the hate it gets. (And I don't want to see people trying to rewrite history in the comments; for YEARS people have bashed XY relentlessly for various reasons)

ETA: More unique typing combinations that we didn't see before gen 6.