Honestly the campaign. I am of the mind that Halo is primarily a story oriented game with some good multiplayer as part of a complete package. I feel like it's so unfocused and relying on "squad gameplay" hurt some story beats. Like why Blue team blindly follow Chief after he goes "yeah my AI GF is sending my visions about her being alive lmao you wanna join my quest"? After his defiance I don't think they should have been cool with it. That's something that just bamboozles me. Also too many new characters Osiris team can and could have been interesting Locke already peaked my interest and Buck as a Spartan. The whole Spartan 4 bad is something I don't personally subscribe to but I can definitely see why the only one I do like is Palmer in the campaign of Halo 4 but she comes off as so stiff in Spartan Ops. Idk I feel like the new characters bring the story down and after the mission that Chief defies the UNSC command Blue team should have tried to stop him and have him run off and make Locke and Buck hunt him that way in co-op Buck could be a second character and Locke could play off of him and we get more personal with the two. I have way too many feelings about the campaign this was too long.
TLDR: Campaign had a TON of potential but was mediocre at best.
Right on, campaign really sunk it for me especially because like you said it had a ton of potential. I wish they had just stuck to Locke vs Chief and had more references and relation to Hunt the Truth, instead of giving away cortana's revival in a bit of dialogue in the second mission and filling the rest of the game with chasing her down while your squads fill the time you are supposed to be enjoying the game with endless exposition and boring banter.
Such a shame, here's hoping Infinite gets it right.