So when things are mixed "improperly" (it's subjective), it's very distracting to me. I don't need to force mono everywhere, but it's very common in amateurish channels, and surprisingly also in movies and TV shows. Big productions tend to mix assuming you'll play on speakers (where it's fine to have something playing on just one channel/speaker, since both your ears will hear it), but when it mixes down to stereo and you listen on headphones, it's soooo common for them to pan something 100% to one channel when the source is supposed to be on that side. Like, somebody speaking to the left of the camera, and it comes 100% on the left channel and 0% on the right one. It's so unnatural and annoying to me.
Exactly, and this highlights the big difference between using headphones and using speakers. When you listen to some stereophonic music with one of the instruments panned completely to one side, through speakers that sound will only play from that side, however the sound will bounce around your room and you'll hear it in both ears, and the difference will tell you where it's coming from. But when you listen through headphones, you don't get this effect, and it sounds weird. With modern computing devices, it shouldn't be that hard to run the music through a filter that mixes the two channels when using headphones to avoid this problem. I wouldn't want to mix them to mono (that sounds bad too), but just a slight reduction of the stereo separation would be good.
I find this genuinely baffling; I lived for nearly a decade as a bachelor in a basement apartment where I had a big TV setup, but out of respect for my upstairs landlord listened to nearly everything on wireless stereo headphones, and I can’t recall ever experiencing this.
Perhaps they're using a weird media player?
I wouldn't know because I consider all those effect libraries, mixers and presets ("Concert Hall" - who would ever want that?) that usually come with the audio chipset driver suite as bloatware and try to get rid of them, or at least never touch them - but it would surprise me if there weren't anything that affects the amount of stereo separation...?