Well, quite simply, the gameplay was modular. Movement and combat occurred on the same plane, which is the base style of any good arena shooter.
Halo’s 1, 2, and 3 all have one max movement speed, and one max jump height that can be marginally increased by crouching in mid air. Duels most often come down to player accuracy with either the Magnum in CE or the BR in 2 and 3. You don’t have thrusters or sprinting to escape, so you have to be more cunning. Crouching, jumping, and of course, strafing, can all help to turn a headshot into a bodyshot and give you time to execute a reversal.
This ‘dance’ is what made Halo so beautifully addicting. Its simplicity lends it to far more variation.
Take this 1v1 encounter and apply it to 1v2, if you think fast, you can beat both of them. Learn to bounce grenades off walls, ‘ninja’ people by crouch jumping backwards over their head, and you can take out 3 or 4 without even using a power weapon. You add in the so called ‘golden triangle’ (Weapon, Melee, Grenade) and you have your base formula. You can then apply variations like verticality, movement objects (man-cannons), vehicles, etc, because it lends itself to said variations, it’s adaptable.
The satisfaction of out-thinking another player is always (to me, anyway) far more rewarding than merely outgunning or outmanoeuvring them.
I don’t get satisfaction from sprinting to hide in a corner and shotgunning the player who was dumb enough to chase me,
I don’t get satisfaction from ground-pounding another player because they weren’t looking up at that particular moment.
I do get satisfaction by dodging someone’s melee by crouching and managing to counter an ambush because they didn’t react quickly enough.
Halo 2 and 3 are thinking player’s games, which creates that skill gap you mentioned. But they also cater to the brainless, when you want to take a pit-stop on the ‘Road to 50’, or if you’re just not interested, there’s always games like Infection, BTB, Juggernaut, Fiesta, Race, Griffball…and the gameplay didn’t change to fulfil these game-modes, the game-modes evolved from the gameplay, which I think is a testament in itself to how adaptable it was. This applies to Campaign and Firefight as well.
Also, this style of gameplay…it isn’t present in any games right now, nor was it back then. Halo’s golden age (halo 2 and 3 specifically) formula was and is completely unique, and no other game on the market offers what it did, which is why even people like me, the more ‘realist’ of the classic fans, people who aren’t blinded by nostalgia goggles and can point to the older games and recognise their flaws, still want the older style back. Because we want 343 to fix the problems with the older style and make it better than Bungie could, heck, Halo 2 Anniversary’s gameplay fully realised would be ideal to me, that multiplayer suite is quite literally what I want Halo 6 (or at least the spin-off after it) to be.
Oh and, MCC isn’t the best exposure to these games, while there is a hefty skill-gap in Halo 2, it would be manageable had you been placed against players of relatively equal skill, which MCC isn’t known for doing…it’s kind of a ‘you had to be there’ thing with 2 and 3, which Is why I find it understandable that newer players just can’t relate…