To be honest, everything outside of the really cool premise. visuals, and the combat itself (which was pretty good).
The story line took a LONG time to get going, as Blueton mentioned.
Outside of a few neat side quests, there was just a stifling amount of fetch-style quests, exploring, and grinding. I don't mind a little exploring here and there, but it was just really repetitive.
Aloy herself, while a total badass, was IMO an exceptionally boring main character compared to the characters around her. She felt like a sort of half attempt to have you play as a character, and a half attempt to have you play as an avatar...and it just fell short in my opinion. Her personality was flat.
To be fair, this game is very heavily influenced by the Far Cry formula - a style which I'm not a fan of, personally...although I will say it does work better in a more interesting scenario like Horizon's world.
I thought the ending to the game was actually very interesting, but the journey to get there was a slog.
TL;DR, this.
Not to be rude to anyone that liked it, of course, but I felt sort of like it was generic action RPG #3 with a really cool coat of paint. I don't think it deserved the hype it got, but that's obviously just one (oddly jaded as of late) guy's opinion