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. 
The Goo faction in Gray Goo is cool as fuck, your "base" is just this big mobile amorphous blob that engulfs and digests resource nodes and even enemies that touch it. It can split into more copies of itself or smaller, faster blobs that further divide into your actual specialized units. Everything is a slick-looking liquid metal with glowing cyber hexagons running across the surface and eerie creaking/bending metal sound effects.
Yeah, petroglyph leaned into it for both Universe at War and Grey Goo. I always felt like the games felt kinda junky, but I appreciated the effort in making the factions feel substantially different.