EDIT: Seems dynamic music is back in style in some very recent games, many of which I haven’t really played yet. Good. 
For me, it’s dynamic music, the kind that some games had that adjusted moment by moment to what was happening in the game.
The best-known example of this in the 90s game TIE Fighter, where the moment more enemy (or allied) ships showed up the music would have a little additional flourish to acknowledge the shift in battle. There were pre-battle tension tracks, battle music, complications of battle, grandiose flourishes for the arrival of enemy or even allied capital ships, and victory and failure music all ready to flow into the next seconds of the game.
A lesser-known but still excellent example of this was in Ultima Underworld and its sequel, where drawing a weapon had its own special “preparing for battle” tension music, getting attacked had a jump-out-of-your-skin joltingly sudden musical start that actually scared me as a kid when I got ambushed, music for battles going well, going poorly, victory and defeat.
I wish more games did those sort of second by second musical changes, but they’ve sort of fallen out of fashion for the most part. 
Most shit still present in TF2 like:
Open chat. I like to get in rivalries and talk shit with the enemy team.
Server setups. It could be a pain to find a good server with room, but it meant you could pick a tryhard server for a while, maybe go back to your usual spot and meetup with the gang, then try some server that solos your favorite map 24/7, before taking a bite and trying out one with crazy
Mods. Maybe somebody had a half-finished mario kart server. Or a randomizer where you got a random gun and attributes.
Server setups need to make a comeback.
Imagine Elden Ring, but you open the server browser and get a list like
TF2 made me obnoxious going over into other games, since most of the servers I was in were basically VR chat where we’d harass anyone for actually grabbing the flag and advancing the game.