In most modern shooters, TTK is usually very high and most guns are able to kill relatively fast. The guns are not that interesting because they all do the same thing essentially despite differences in range/damage/accuracy, you can reliably kill with them on their own. and you usually spawn with them regardless.
In Halo, the most simple delineation between the standard weapons and power weapons. The power weapons are simply better when used situationally, although they have more limited ammo. Simply having the weapon doesn't mean you're going to annihilate, but if you use them in the correct situation, they will usually destroy anything else. They usually spawn in certain places on the map. To take advantage of these, you need to have good map control to make sure you can always be in a position to get them. This adds more strategy to the map, so instead of a simple tdm you now have to think about where exactly you want to be to exploit this. Positioning is much more important in Halo than it is in its mainstream competitors.
Amongst the standard weapons, it isn't simply a choice of picking one gun and using that whilst relying on a sidearm as an out of ammo option. Weapons can work very well together. For example, the Assault Rifle is good enough at bringing down shields but not accurate enough to land that last headshot when the shields are down. Pair that with a Magnum, and now you can burst the shields down and quickly finish them off with a well placed headshot. Similarly, a plasma pistol isn't going to be getting you kills on its own, but its overcharge can instantly take down shields, leaving the enemy vulnerable again. To me it is much more satisfying to use these combos rather than just shoot someone a couple of times and watch them fall over.
As the TTK is much higher, you often have the opportunity to fight back when shot in the back, unlike most other shooters where you wouldn't stand much of a chance. Personally, this removes the frustration of being spawn camped by a flanker in games like battlefield and means people can still flank but must also be skilled enough to follow through. In other words, fights are always more skill based and you actually feel like you always have a chance.
This is why I think people dislike Halo 5 so much as a Halo game. The TTK is much lower in that game, so people don't need to use the combos any more, and the ads system only makes that worse as guns like the AR can be far too accurate. As the TTK is lower, power weapons are also less valuable as people can kill you faster with the standard weaponry. People do enjoy Halo 5, and it is good if you enjoy a more fast paced simpler Halo, but it isn't the gameplay that core fans expected.
These are just my opinions about the game, i'm sure someone who has played more can give you a better idea but these are some of the reasons why I think it is unique.