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. 
SSX 3 is one of my favorite all-time games and the dynamic music was one of the reasons. Not only is the soundtrack full of amazing artists and songs, elements of the tracks duck or swell contextually. As you soar into massive air tricks, the lyrics drop out and then the music crescendos when you nail the landing. When you glide through an ice cave, the leads and highs hush and the bass and rhythm push like an underground river until you reach the surface and the other tracks rush back in. It’s great.
That was just a perfect game