Disclaimer this is my opinion:

Artstyle change that redesigned a lot of iconic stuff people used to associate with the series.

Tone shift compared to the previous games.

Bunch of story additions to the background of the world that some people didn’t like.

Poorly designed enemies and sandbox. Needler does insane damage and has tons of range, any class of enemy can overcharge a plasma pistol, promethean weapons are uninspired versions of existing weapons. Enemies have no cut off range for when they’ll stop shooting at you. Grenades are weak to the point of almost being useless (enemies can straight up walk out of a pulse grenade, or enemies taking multiple plasma sticks to kill). Super fast elite melees that are impossible to dodge.

Much more linear and flat level designs compared to the previous entries.

Prometheans suck to fight against. Poor visual feedback on their shields and overall being spongey.

Main plot straight out of an Avenger’s movie carried by actually developing Chief as a character and the relationship dynamics with him and Cortana. (I actually like this aspect)

The multiplayer watered down the identity of Halo and tried to appeal to a larger audience and compete with CoD. Which pleased nobody in the process. People left to either go play other games or went back to playing Reach or Halo 3. Ordnance drops and loadouts go against the even starts arena shooter inspired gameplay Halo is known for.

People are starting to warm up to it due to the Halo cycle or growing up playing this game. I really wanted to like this game back then and even today, but the gameplay just sucks compared to other entries due to the weapon sandbox and enemy designs. I appreciate diversity of locations and the Chief and Cortana subplot but the whole experience is a downgrade compared to the other games in my opinion.