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. 
this one was never really popular, but hardware gimmicks. from the light gun to dance pads, going through wii motion controllers or even double screen of the ds and wiiu (fuck you nintendo for abandonning the wiiu) I really miss having different ways of playing games that was not only game pads or keyboard + mouse. even if the games were not great, having some physical interaction with them was fun to explore. sadly, even when they were "common", not many games made very good use of them, resulting in most games being mediocre at best
the closest modern equivalent I can think about is VR and it is not something I want to play with
Wiimote was so good. I loved the point and shoot in Resi4 and the sword and shield combat in Twilight Princess. Legit better than any VR I've done
The wiimote's accelerometer-based motion controls weren't very good, but its IR camera pointer controls were fantastic. It's a shame motion controls became synonymous with the former instead of the latter.
the wiimote accelerometer improved greatly when they added the wii remote plus attachment. sadly, I can only remember 3 games that actually used that capability (and one is from the wiiu) but their control scheme was really great. it was a shame that they released the plus after the community decided that motion controlls = bad so nobody wanted to develop games using them