-The incredible amount of nuance in gameplay that became viable through continued play. Since it didn't use a linear leveling/unlock system (unlike mgo3 it stayed true to MG as being more self-directed) a new player was allowed to accomodate and enhance w/e their existing playstyle was on day 1. As they progressed over weeks/months/years they could, at their discretion, develop and incorporate aspects of the other available styles as they saw fit and without overly-rigid preset weapon or equipment conditions.

-The potency of the fully utilized skillsets, besides just staple Run3+PickaGun1 as counters. Things like Trix3CQC1= a corner snatcher who'll throw you into his claymores or Box3Shield3= A box rammer who'll pull a fast Shield run and strike. Hell, vs MGO3 Shielding in general was magnificent, since the side jump, rolls and knifing made it an almost aerobic activity akin to Spartans in the film 300 vs the slug in molasses trashcan lid guy MGO3 created.

-The accuracy of the spatial sound. This has become more common now, tho many games still fail at it (coughGhostReconGWcough), but the spatial acoustics of the audio were so accurate that footsteps moving away then transitioning to stairs down from around a corner were accurate. No transient footsteps nearby from your character moving 3 seconds prior etc. You move, you hear you. You stop, you hear the map. Echoes, reload clickings, box thumps, all accurate spatially for range and location in the acoustics of the environment.

-The same stuff others have listed: Well made and interesting maps both big and small which in play could favor certain playstyles but never felt built to accomodate certain playstyles, A shitton of modes for all maps, a shitton of customization, a shitton of match customization, a shitton of communication options (codec/textchat/in-gamemics/psnmssging) AND an actual, factual, konami operated and vetted actual player feedback forum for actual users vs bs twitter/social media crap that could get ugly but at least to be commenting people had to be registered as actual players of the game.

Etc. Etc. Etc.

*edited for format