The problem with Halo 4, according to most players, is that 343 attempted to broaden the demographic by adding gameplay mechanics such as loadouts, while also changing the artistic style (343 has done a lot of research on Asian art which influenced the armour designs, and they took some punk/techno references for the environments), and adding yet another layer of complexity to the story (people don't like how Bungie/343 have been just throwing more Spartans at them, and the Forerunners have lost a lot of their appeal now that we know they're generic-Covenant-types).
A lot of players don't like some of the changes 343 made to combat encounters and the weapon changes.
Personally, I prefer Halo CE - Halo 3 mostly because of the style.