If you've played previous Halo games you might be aware of little subtleties which people are complaining aboutthat make it similar, but not identical to COD
1. Customisable Loadouts - never ever ever ever ever ever ever been present in a Halo game until now (Reach had preset loadouts in selective playlists)
2. Reduction in Health - In previous games, it would take more than 3 shots to kill you
3. Killstreaks - Ordnance drops are a killstreak similar to Care Packages in COD where no killstreaks have ever been present in a previous Halo
4. Ability to leave/join whenever - very similar to COD, previous games would go to the extent of punishing you for leaving a game
5. Instant Respawn - COD has instant respawning, Halo has always had a little countdown which increased after a suicide or betrayal
6. Perks - An exact copy of COD. They've even taken "Warlord/Overkill" (2 primary weapons) and "Marathon" (infinite sprint) and put it right into the game
7. Bloom - Previous games would punish players for spraying by making the reticule "bloom", making the weapon less accurate. Halo 4 has very little to no bloom at all
8. Killcams - This isn't necessarily a bad thing, but it is similar to COD
9. Levelling System - COD has a straight 50/60/100 or however many levels, almost solely based on play time. There is no skill ranking system. Halo 2 and 3 both had a system where the better you were, the higher level you could achieve (this was removed with Reach)
10. "Prestige" - Like COD, Halo 4 brings with it a type of prestige system, where you can restart after the highest level and pick a "Specialisation"
11. Unlockables - Weapons being able to be unlocked based on level is a copy of COD. Halo has never had unlockable weapons and this ties in with the customisable loadout point. Unlockables mean that someone at a higher level has a selective advantage due to the fact that they could potentially have better weapons
For those 11 reasons, people argue that Halo 4 is a new copy of COD. Unfortunately, I have to agree that a lot of aspects are taken or borrowed from the game and they definitely detract from the Halo experience which used to have balance and be based off skill. However now the levelling system is becoming a 2,000 game grind and the combat experience is about who has a better loadout.