This video goes over it fairly well (note that this video does contain minor spoilers): https://youtu.be/O-x-1kS6TpQ Another video that goes over only the gameplay with no spoilers: https://www.youtube.com/watch?v=oBv8UVOSG8U

Spoiler-free TL;DW: Halo 4 was controversial because this was the first new mainline Halo game by 343 Industries, who changed things up from previous Halo titles.

But aside from questionable changes to the franchise in order to appeal to the CoD audience, there were a few genuine issues with the campaign (in my opinion, as a player who never finished Halo 4’s campaign).

In terms of gameplay:

1. The sandbox was absolutely destroyed through many changes to older weapons, to encourage you to use the new 343 weapons. The weapons were either nerfed into the ground to make them unviable, greatly reduced in ammo/availability, or outright removed. This meant that you would constantly be forced to replace your weapons, or you’d have to use the newer weapons, which still ran out of ammo. This greatly limited your options when playing the game.
2. The enemies would act in a way that greatly limited your options when fighting them. The most common weapons (plasma pistols and needlers) were both were used in a way that forces you to methodically take them out from a distance. Melee attacks were undodgeable instant deaths, forcing you to take them out from a distance. Then, the Promethian knights have stupid health, shields that recharge almost instantly, and the ability to fucking teleport to recover their shields (or hit you with an undodgeable melee attacks).

The result is that you are basically limited to only using long-range weapons to take out enemies. Hope you enjoy hours of Light Rifle gameplay!

In terms of other parts of the game, this is more subjective but:

1. The story felt incoherent, especially considering that it is a sequel to Halo 3
2. The sound design completely changed the iconic sound effects, and the music felt off
3. The characters were bland and forgettable, only Master Chief and Cortana were anywhere near fleshed-out

Overall just a poor experience compared to older Halo titles.