The campaign to be well told in both the story and how the gameplay explains the game itself in a long and extended campaign. Halo 2 and ODST did well on this, having some of the best campaigns in the series. When you think about it, Halo 2 was the first campaign to allow you to switch characters to explain different sides of the story. In doing so, it gave both main characters a lengthy amount of time to give a depth of character to both. Granted Chief at the time had jack all to his character, but his role in his fight against the Covenant and later the Flood was explored in a 'final stand' sense. The Arbiter on the other hand was given backstory before the gameplay even started and showed a large change in character as the campaign progressed.
ODST was similar in the sense that the characters were given their own short stories, with Rookie searching in a dark, desolate and quiet metropolis. The detective style of gameplay obviously isn't everyone's favourite way to play a game, but it's undeniable that way it was presented allowed the pretty poor story in ODST to seem so much better just by playing in such an unusual style.
Halo 5 Guardians failed in the style that Halo 2 conquered, the story is left up for debate for who likes it. However it's excusable on how short it was, the story given was presented in such a lazy way with missions that if removed from the game would've left very little difference to the campaign. The first two missions in particular, if those were removed it would've had absolutely no impact on the game at all. Dr Halsey has no impact on the story in any regard other than confirming what Lasky already knew, Chief is chasing after Cortana. Argent Moon also has zero impact on the story, that Covenant fleet had no backstory in any official material 343i has stamped nor was given any mention thereafter in the game. Blue Team could've easily had a mission that had any relevance to the story other than "I'm defying your order, AWOL now".
Additionally the entire story arc on Sanghelios was completely pointless, the only reason you played on that game was because it was fanservice from 343i just to get you on a Guardian after spending missions doing crap all. Kraken fight? What's the point of it, you're given no information to what it is doing on the planet nor what relevance it has with a Covenant assault force. That entire arc could've been scrapped for story that was related entirely to the Guardians, which is the title of the game, not Halo 5 Sanghelios, which albeit sounds pretty fun.
Moving on from the actual missions that are 'related' to the main story, the gameplay was atrocious. When you meet and fight the Warden for the first, maybe even the second time, there is a sense of "wow, now this is a boss that seems cool for Halo". Actually having a mechanic on how to kill him is rather different to how we've fought previously in Halo and ties in well with the new mechanics for Prometheans. Then you meet him for the third time, then the fourth, oh boy here we go, then the fifth, then the sixth. What on Earth was 343i thinking? Answer, they weren't. The pathetic excuse of "He has a single mind but a million bodies" was such a lazy way to excuse the fact that 343i could not think of varied combat environments without having to resort to the wave after wave of Promethean forces. When thinking back on it, the only fun missions in H5G were the ones that didn't even have Prometheans, because the combat was actually different and fun due to the way the Covenant work. I thought Halo 4 was boring in how the Prometheans were handled in a rinse and repeat environment, but at least it had one fun mission being in the Broadsword. H5G's was just laziness and uninspired environments that seemed to be copy pasted everywhere in terms of gameplay design.
Then you have the characters, Blue Team was completely and utterly wasted. One of the favourite set of characters in the extended universe put into the game, for what? To stand there for a few missions with zero impact on both the story and Chief's character? They had no character development and explained nothing about them while out of combat. For me the only purpose that they served was for getting Linda's Sniper Rifle. Speaking of teams, Osiris was a disgrace. You play through 12 missions with them for you to only get a backstory on each character in the final mission from Cortana going through the Service Records. I'm sorry, but that is not how you explain backstory on any main character regardless of their importance to the story. When you introduce a character, you introduce, the character. They were empty shells (other than Buck) for 93.3% of the game, then to have their Service Records just read to them as a last ditch effort to explain who they were. It's like 343i completely forgot that they didn't explain who anyone was and had to find a crappy way to give any ray of light on a character by that point. Hell, even the corrupting AI on a colony world had more backstory and character than 6 of the 8 main characters did for the entire game. Buck and Chief were the only characters that had anything known about them from playing the games from Halo. Buck had been in 2 previous games and Chief obviously being in every main title. Everyone else, whelp you're not getting any backstory about them from playing games alone.
I could go into depth about how poor the basic functions of multiplayer are, or the 'reward' system works, or even how you literally need XBL gold to play solo Custom games. Such poor decisions by 343i have negatively impacted the most fundamental qualities of Halo in such a heavy degree that if you compare H5G to any other Halo game, they feel nothing alike. That being said the actual gameplay for H5G feels similar to the original 3, but that's it. The maps are poor, the lighting isn't good and the hit detection is not on par with what Halo should be by now. These issues are incredibly easily fixed though, just look at the original trilogy maps and see what made them great. Look at Halo 3 and see how well lit everything is. If you're going to decrease aim assist, actually ensure that you're rewarded when you do hit shots then.
These issues are so much easier to fix compared to the issues of the campaign listed above. The only way I can see 343i fixing the campaign is by replacing staff that worked on it. They failed two main series titles in a row, they showed what their best is and it's clearly subpar for what Halo should be expected of by now. Halo's campaign should be grand in scale and indepth with story, and Halo 4 failed on the scale and H5G failed in the story, if things remain the same, Halo 6 will fail in both.