Long post coming. Feel free to skip if you got a short attention span.
Locke has plenty of potential, a cool voice, a strong presence, and top 3 spartan armor that I've been trying to unlock since day one for multiplayer (Emile from reach is up there with him and the chief), but they explored his motivations so poorly that it made him unrelatable. Halo 5 gets hate due to its changing of the art style in my opinion for one. The elites look and walk and talk more like brutes now.
There are too many Spartans now. We went from the chief being the last one left in halo 3 to now there's at least 9 between palmer and the two fire teams. This also meant no marines you fight alongside in the campaign too.
The weapon sounds changed, most notably the battle rifle, the pistol, the shotgun, the sword, and the rocket launcher. They changed the look of weapons when people didn't want them. The storm rifle, the battle rifle, the fuel rod cannon, and the smg. Aiming down the sights and sprint and thrusters and clambering and the charge and ground pound are cool features, but they don't belong in a halo game. This makes it feel more like call of duty or destiny than it does halo just as much as the change of art style does. There was no reason to change what the scorpion and wraiths looks like.
The campaign was a mess too. Who is the main character? Locke never feels like that guy, but you play as him in 12 of the 15 missions. The warden isn't really the main antagonist is he? He exists solely to prolong the campaign. You fight seven wardens in the campaign. The levels are too linear with no room to explore and be amazed at the scenery like you were in the first three and even in halo 4 to an extent when you were outdoors. You have to read way too many books and watch too many short films separate from the game to know who these people are and what's going on. None of the characters get fleshed out because there are too many of them. Even the dialogue is bad in this game compared to how people talk in the previous 4 games. Halo had the chief, Cortana, captain Keyes, and Johnson. That's it. Halo 2 had chief, Johnson, Cortana, Keyes, the arbiter, Tartarus, the prophets, and hood while managing to flesh out all of them. Halo 3 had chief, Johnson, Keyes, the arbiter, and truth. Halo 4 explored the dynamic between chief and Cortana. Laskey got a little bit of depth at least. Back to halo 5 now. Who in the campaign did anything of importance or felt essential? If you removed palmer or Halsey or Roland (the AI of the infinity) or the arbiter, what does this game miss out on? If you remove buck and vale and Tanaka from fireteam Osiris and remove Fred and Kelly and Linda from blue team (once again buck was the only one who made his debut in a halo game as opposed to a book or movie), what does the game lose?
Does anyone like the idea of Cortana being the villain now? Halo 4 had its problems but I thought it told a great story and that Jen Taylor really knocked it out of the park with her performance in halo 4. This cheapens everything her character has done to this point. Why not have jul mdama be the main bad guy instead of kill him off in the first level? Why is so much of the story explained in dialogue between the characters in levels instead of cutscenes to make things carry weight? Cutscenes should never be action sequences like the openings of each of the first two levels?
Why are the prometheans still here and what are they fighting for if the didact is gone (side note I'm still skeptical that he's dead. We saw him fall into that thing but we didn't see him die)? Worst of all, is that the marketing of this game lied to us. They gave us the okie doke, the hesitation stop and go move, the double move on the post route, the UTEP two step killer crossover.
It was marketed around the conflict between Locke and the chief. That he had gone awol or turned and Locke was supposed to hunt him down and we didn't know the backstory behind this to add to the mystery. We weren't supposed to know what the deal is with the guardians so early in the game and chief wasn't supposed to go absent in such an anticlimactic fashion. The final mission didn't have an exciting climax either. Think about what all the ads gave us and what we actually got. Locke was supposed to be boba fett chasing his target and instead we got a short conversation with a poorly coordinated fight and then right afterwards it becomes a rescue mission to save the chief. 343 really fukked up the campaign badly. Go back and play the campaign of the first 4 games and see how much better each is in regards to the story it tells. I'm almost certain at some point they switched up what they were doing late in the development of the game. No other explanation makes sense.

if you stuck it out and read this whole thing.