While I don't agree with the OP that Infinite took all the fun away, there are certain minor inconsistencies in the sandbox implementation as compared to previous Halo games that keep it from being as great as it could be. That said I've always thought Halo was the best fps implementation, in agreement with the OP.
-No friendly fire changes the dynamics and risk assessment of throwing grenades, one of the primary combat 'tripod' legs. No longer do you have to weigh the option of throwing a grenade to aid a teammate for fear of also killing them. Are they in a capture zone? Throw everything to got to help keep pressure off of them.
-Melee hit detection is wonky, as is the lunge. To be fair the range and stickyness of the lunge seems to change with every iteration and it takes some time to get used to it. But, there is something off with the hit detection. I've gotten, and been victim of, one-hit melee kills to the front, as if it was an assassination from the rear.
-I greatly feel the loss of Reach's 'shield threshold' mechanic. For the unfamiliar, there were few instances (1hko weapons primarily) where damage would 'bleed through' shields and into health. When the shield popped, so did any overkill damage from the attack that took the shields down. What this boiled down to was a very binary point, if shields are up a melee attack will only take down their shields, if their shields are down, it'll finish them off. Coupled with a very clear visual indication when someone's shields went down, it became an instinctive part of the flow of battle, to me.
-No collision with teammates is pretty awkward. Several times already I've had someone walk through me right as I fire a rocket, which explodes on the back of their head, killing me. At least with friendly fire they'd die as well and maybe learn to stay clear of the guy holding a damn rocket launcher.
-The battle rifle feels wrong. I can't expand on that, it just hasn't gelled with me yet.