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. 
They're a lot harder to make with modern / 3D games.
But they also seem to boost a game's long-term audience by a lot, so it's weird they're not more common.
...I should make something that needs a level editor instead of always doing procedural stuff.
I'm no expert - what makes them harder with 3D?
From a layman's perspective isn't it just giving your users access to a bunch of assets - I suppose streamlining the creating process to be user friendly is the difficulty. I can make a custom Far Cry map no problem, but GMOD or Skyrim I was at a total loss with their creation software.
FarCry does a pretty good job. I don't mind if it's all a bit janky and you have to find creative workarounds to problems. GTA V seemed to also have a lot of good custom maps.
That's pretty much it, it's a push-pull between streamlining for sane folks and raw power/control for insane folks. Also the first one takes extra dev time since devs are insane by definition and so are already using the raw tools.